First you hit the wall and then you make it to the other side. Adrenaline and cheering crowds pull you across the finish line and after completing your first marathon you bask in euphoric exhaustion, a sense of achievement and quiet pride about the cash you have raised for charity.

Everyone has those cliched imaginings echoing in their heads when they set off on their first marathon. I did when I ran mine last year. After his heart stopped in 1990 - not while running but playing polo - the late Australian media mogul Kerry Packer recovered and famously quipped: "I've been to the other side, and let me tell you son, there's fucking nothing there."

And so, dear novice runner, what is really on the other side of that legendary wall you hit when running your first marathon? Nothing. Well, nothing good. Only pain, grief, heartache, tears and fury at all those platitudinous, positive-speaking marathon freaks who conned you into believing it would be the most fulfilling thing you could ever do.

Don't let me put you off. If you're running London, you are by now a prisoner of your preparation. Physically there is not much more you can do, apart from rest and eat properly. Mentally, however, you can still make a difference. Take it from me.

I've jogged since I was a teenager. I trained sensibly, I warmed up, I paced myself, I ran the whole 26.2 miles, I hoped for under four hours and in the end finished in 3 hours 45 minutes. I raised more than £1,200 for charity but I felt wretched. My enduring memory is of a horrible day.

Beforehand I was queasy with nerves. During the first eight miles I had to stop three times to go to the loo. It was cold and very windy (my marathon was Edinburgh, not London). After eight miles I pulled a calf muscle.

I'd never run in a crowd before and I didn't like it at all. I felt completely alone. Other runners jogged along in pairs, chatting. They didn't look as if they were in agony. A few spectators shouted words of encouragement, which was nice. But a negative competitiveness took over. I wasn't used to people overtaking me and here, everyone was. I felt weak and useless. At 16 miles the pain got worse.

I clung to the 3:45 pacemakers (they had flags on their back and chatted to each other incessantly in Danish) for another six miles. Then I saw my girlfriend and two friends cheering me on. They looked fearful. It was like seeing a skeleton, they said afterwards. Once I'd seen them, I could barely keep going. The pacers powered on. I shuffled forwards, speeded up for the final mile, finished and felt desolate. I wanted to cry. I think I wanted mummy too. Life seemed an empty globe of awfulness, with me at its pathetic core.

A year on, I talk to Dr Victor Thompson, a clinical sports psychologist who works with athletes and amateur runners in London. It seems that, psychologically, I was woefully unprepared. Firstly, you need to have dark experiences in your training, come through them, and hold them up as examples of how you can cope. I kept calm when I pulled my calf muscle (I'd done that before in training and knew I could keep running) but wasn't familiar with those negative feelings that flooded in during the marathon.

"Some people find it beneficial to think of the race in chunks. When you expect the difficult periods you're not lulled into the thought that when it hurts it is going to hurt forever," he says. "It can be common to go through a range of emotions, even if you're running well. Even elite athletes have lulls or periods of doubt. You need to use your experiences so whatever is thrown at you turn it around. Store what has been thrown at you in your dark moments in training and they will be familiar to you during the race."

Thompson also suggests that you can remind yourself of the bigger picture and the reason or charity for whom you are running the marathon: your pain will be nothing compared with the people you are helping. I tried this rather smug mindgame but it didn't work for me. So in a moment of crisis I seized upon another, much less worthy, idea: surrounded by very short young women, whose legs must have been half the length of mine, I told myself I was pathetic if I couldn't keep up with them. Thompson is not impressed. "If you are then passed by the short-legged women that might be soul destroying," he cautions.

Dr Mike Loosemore, a sports physician at the Olympic Medical Institute, says experienced runners practise a form of visualisation to take their minds away from physical pain. "We talk to endurance runners about having a place they can go in their head, a happy place, and they get used to putting themselves there - memories of a beach holiday perhaps - a positive image of a pain-free state."

Other mind games you can try are distraction games (spotting red socks, counting red vests) to take your thoughts off physical pain. You could interact with the tunes on your iPod or, better, interact with the crowd. If you can wave at them, you'll get some big cheers back. Or you can give yourself rewards: if jelly beans work for you, give yourself one every two miles after the halfway mark, for example, and if you guess the colour right then reward yourself with two.

I would recommend that if you have any competitive urges and have never run in a crowd before, run somewhere busy - in a shorter race or any place where people will cruise past you - before tackling a marathon.

"When you get overtaken by six vikings carrying their own boat it does take you down a peg or two," says Loosemoore. "You've got to prepare yourself for that before the marathon. The real battle is against yourself. You are going to be overtaken. There will be extremely good marathon runners in rhinoceros costumes. Try not to be distracted by that."

Lots of runners get emotional at the end. But like drunkards, not everyone will have the same kind of hangover. The emptiness I felt for several days afterwards may have been because I expected to do better after a lifetime of enjoying jogging; it may have been that I was generally unhappy at the time; it may simply have been a bad day's run.

Thompson's advice? "Book something in," he says. Here, at last, I had done something right: I went for a curry and then on holiday. "You've had a big goal for a long time and so it's really useful to have some way of celebrating your efforts, even if you've had to walk the last five miles. Bear in mind what your goals were for the race, but if on the day you faced bigger challenges because your body wasn't willing or you strained an ankle, you've got to accept that's the way things sometimes go. Go to a nice restaurant, give yourself a bit of holiday and don't commit to any other marathons for a while."

I've followed that piece of advice as well. Now, of course, I am glad I've done it. My biggest fear was that My Marathon Hell would spoil running for me. But I enjoy it even more now: savouring every step that isn't part of a marathon.

· Five ways to recover from a marathon