The morning after The Observer Christmas party, something seemed different from previous years. Lying in bed, Axminster tongue, head throbbing, replaying sequentially the evening's events, I would appear to have behaved myself. Of course, things got a little fuzzy after midnight but, no, there were definitely no alarm bells ringing. I could walk into work safe in the knowledge that, if there was somebody at the party who had set the bar for idiocy, a giant, sozzled fool against whose antics we could all offset our own misdemeanours, for once it wasn't going to be me.

So, later in the office, when my editor walked up to me and asked when I would be departing, accompanied by a mysterious little flick of the wrist, those hitherto silent bells started to tinkle like windchimes on a gentle breeze. When another colleague approached, eyes like saucers, and shook my hand in the wild manner reserved for serious congratulations, I had a full Westminster-Abbey-after-a-coronation going on. The motoring editor told me that the journey I was about to embark on was 'brave' and the 'stuff of fantasy'. I urgently needed to sit down and revisit the witching hour, crawl back through that Stella-fuelled fog. Oh God, what had I said? What had I done?

I didn't have to wait long to find out. Apparently, in that euphoric place where everything's possible, I had announced that in the summer I was going on a six-month motorcycle road trip. And not just any old motorcycle, but a big one, specifically, a BMW GS1200 (this had required no imagination on my part: I had just read Long Way Round, Ewan McGregor and Charley Boorman's account of their trip across the globe, and this is the bike they rode). I was going to take off around Europe, give up the grind of London life, go where the road took me, have adventures. Oh, how glorious it would all be, I had said - a sentiment shared by the travel editor, sober and enthusiastic, who had declared, in front of many equally enthused witnesses, that I was to write about my adventures in a weekly column.

Alas, what I had neglected to share with my colleagues was that I hadn't been on two wheels since an inglorious three-month teenage chapter involving a Lambretta, four crashes and an 18-month ban for drink-driving. Cliche fans might also like to note at this stage that I am 42, with a receding hairline and a very recent decree absolute.

So, I was in a pickle, and no mistake. The last thing a man having a midlife crisis wants to do is admit that he has been writing cheques with his drunken mouth that he has no way of cashing. The type of people who just take off on a motorcycle can perform heart surgery with a coathanger or decide to join the Foreign Legion for a laugh. They do not consistently get on the Victoria Line going the wrong way.

However, interestingly, since word started getting around of my proposed trip, my stock seemed to have risen, especially with men and women of a certain vintage, as if I had become an unwitting poster boy for the disaffected middle-aged. Further, if in lager-veritas I had been attempting to reclaim a waning virility, trying to spice up a life that had become little more than a sleepwalk through the shadows, then how to backtrack with any dignity? I might as well knock up a sandwich board reading: 'Loser, with hairy nostrils, a broken marriage, no kids, brittle confidence, stagnating career, bad back and now no bottle.' As Tim from The Office said so eloquently: 'Form an orderly queue, ladies.'

I decided I had to go for it. 'The moment one commits oneself, then providence moves too,' wrote WH Murray in his book about the 1951 Scottish Himalayan expedition. And so it was that I discovered that BMW has a residential training school just outside Neath, South Wales, where, in the space of five days, they will take you through your compulsory basic training on 125cc Hondas, then on to BMW 650s for your Direct Access course. This leads, on day six, to your full motorcycle test. I signed up.

Kevin Sanders, who runs the school, holds several world records for motorcycle adventure touring and also runs Globebusters, a company that organises guided motorcycle trips around the Americas. His knowledge of everything motorbike made me tentatively confident that, with a fair wind, I might even make it as far as Dover. The school's training routes take in the Brecon Beacons but are based in the town of Ystradgynlais, which is light on vowels but heavy on pensioners crossing the road without looking. Very handy for practising those emergency stops.

On the day of the test, the words of WH Murray came home to roost. 'Hello,' said my examiner, offering his hand. 'My name is Mr Pass, Mr Pete Pass.'

'Is that really your name?' I asked, looking for the cameras.

'Yes,' he said, deadpan, in the manner of someone who probably hadn't heard that gag for 10 minutes or so. 'Is there something wrong with it?'

There wasn't. Thirty minutes later I was ripping up my L-plates and heading back to the training school for an appointment with my new motorcycle: a staggeringly gorgeous - and staggeringly enormous - gun-metal grey BMW GS1200 complete with the aluminium panniers that would shortly be containing my entire world.

After a couple of days' familiarisation and extra training, Kevin bade me farewell and I was off on my own for the first time, heading for London 200 miles away, wobbling along the A-roads somewhat circumspectly, frustrated tractor drivers stuck behind me. I felt euphoric and terrified in equal measure, fully grasping the popular motorcycling saying: 'Only a biker knows why a dog sticks its head out of a car window.'

On that rain-soaked journey, I had time to reflect on what had brought me to this point. Ten years previously, I had met a woman. Within days, we had moved in together and for the next six years I lived in unmitigated bliss. She was the best friend I had ever had, the person that made me - are you still there, cliche fans? - wonderfully whole, throwing light into the darkest corners.

That she was stunningly beautiful only added to a peachy sense of self-satisfaction that life wasn't treating me too badly. Shallow, but there you go. I used to watch her sleeping and will her to wake up, as if any time away from her was wasted. We got married. We were going to start a family. I had the perfect life.

Then she left me. There weren't really any rows, no slow decay, just a flick of a switch; memories of my marriage frozen for rose-tinted eternity like an untimely death. In desperation, I gave her Yeats's 'He Wishes for the Cloths of Heaven', but I may as well have given her the instructions for the dishwasher. She didn't tread softly on my dreams. She Riverdanced on them. I learnt then that when a woman has truly fallen out of love with you, there is no way back.

So where could I go while I was waiting to fall out of love with her? Well, there was Threshers, and I thank them for all their support during that difficult phase. Then there were the big Himalayan mountains to climb, the marathons to run, the endless hours lifting weights in the gym; anything to eradicate the pervading sense of emasculation, the feeling that, as a man, I had failed. There were the messy fumblings with other women that always, frustratingly, felt like an infidelity, and a terrifying trip to the land of misogyny - and, boy, is there a big ol' gang of guys living there willing to recruit you to the cause.

I didn't recognise myself any more. I was utterly stuck, approaching 40, watching as all my friends had babies. I felt prematurely old. My courage was gone, my shoulders hunched. I knew that tomorrow would be just like today. Bitterness had me by the throat.

Most of all, I filled that time with reading. I devoured everything I could get my hands on about relationships, trying to find some peace, some answers. Slowly, as two years' separation turned into three, it dawned on me that what was keeping me stuck was not just the failure of my marriage, but the fact that it had happened as I hit middle-age.

I came across a quote from Saki that hit me like a thunderbolt: 'The young have aspirations that never come to pass, the old have reminiscences of what never happened. It's only the middle-aged who are really conscious of their limitations.' It was true. I had brutally lost my idealism, yet was unable to exercise any pragmatism or see the future optimistically.

The answer was as clear as the hairs sprouting out of my ears. I had lost my way. But the way forward wasn't to cling pathetically to the vestiges of youth like a drowning man to a life raft, but to embrace the afternoon of life, wherever and whatever that might be.

I needed to travel again; to find out what goes on in the afternoon (though hopefully not Hemingway's version). I needed to travel not with the naivety of youth, when I was genuinely surprised to see my reflection staring back at me in the mirror at the loo at JFK Airport, nor with the safety of a package holiday, but by exposing myself to risk and failure. All that latent desire needed to burst forth was the requisite combination of Stella and a bunch of witnesses.

I have let my flat for six months and remortgaged it to pay for the trip. I don't know where I'm going - turning left at Calais is as far as my planning has got - but such a directionless schedule seems somehow apposite.

In all probability, I will head over the top of Scandinavia, then through eastern Europe and a big loop of Turkey, back through the Balkans and home in time for Christmas.

Sitting here writing this, days before departure, I will confess to feeling anxious. This anxiety is not about riding a motorcycle, or the impending isolation, or bandits, or breakdowns, but something else altogether. It's a dread of the unknown I've acquired lately that I cannot quite throw off. But this is exactly why I have to go.

Essentials: Mike Carter trained with BMW Rider Training (08000 131282) and Globebusters (01639 843526). His camping gear is from Cotswold Outdoor (0870 4427755) and panniers from Metal Mule. His crossing to Calais was with SeaFrance (0870 5711711).