Eight-and-a-half hours into the 100km (62m) Norfolk Coastal ultramarathon, the pain in my legs and badly battered feet was almost overwhelming. I desperately wanted to give in to the urge to curl up in a ball at the side of the trail and shut my eyes.

Sixty-two miles is not particularly far in the ultrarunning world – and the bleak, beautiful and flat Norfolk coastline may not be as hard on the legs as the Alps or Hard Rock 100 – but it was further than I had ever run before. The race pounded me almost into submission before I broke through and was lifted on a wave of euphoria unlike anything I've ever experienced.

Writing this a few days later, the beatific feeling is still there – an almost spiritual high that has left me drained mentally and physically (I came down with a three-day cold in the 'open window' immediately after the race), but satisfied. One runner I spoke to described it as like the layers of your self being peeled back one by one until all that is left is an irreducible core.

Ultrarunning is a personal journey, and the revelation for me in my first ultramarathon was the use of a positive image to spur myself on when the going really got tough, rather than a negative one. While I often use positive visualisation in the build up to a marathon, whenever I've felt like quitting – or wanted to run faster when there's nothing left – I've pulled out some skeleton from the well-stocked cupboard of my childhood and rechannelled the anger into forward motion.

Using the 'dark side' had felt cathartic – as if I was performing some kind of alchemy in transforming messed-up emotional crap into endurance running gold (I wish). But as I crunched over the final shingle stretch of Kelling beach and headed up the hill towards the finish, I conjured up an image of my six-year-old running alongside me – laughing and joking as we often do on our Sunday morning jogs around the local park.

In the end I surprised myself by placing second in 9 hours 40 minutes.

I've run a few marathons and regularly knock out 40-plus miles a week, so I'm no stranger to runner's high. What made my ultra experience so much more powerful?

Since the 1980s, endorphins (opiates which are naturally produced during strenuous exercise) have been credited as the cause of runner's high. But recent research has claimed these molecules are too large to "pass the blood-brain barrier" and affect the mind.

The search for an alternative has focused on endocannabinoids – natural substances produced in larger quantities by the body during exercise, which are chemically similar to the active ingredients in marijuana. Other possible candidates include epinedrine, serotonin and dopamine.

Tim Noakes, professor of sports science at the University of Cape Town and author of Lore of Running, says academics have "no idea" what causes the high. But, he adds, "We all know what it is if we have ever experienced it – I touch heaven when I develop it."

Noakes believes the high is stronger in an ultra (he says his best have been after a few hours of running), but he is not worried about addiction. "It makes you feel so good – but after four to five hours you are also about to get really sore, so this prevents it becoming addictive."

Scott Jurek, the legendary ultrarunner and seven-times winner of the Western States 100-mile run, writes in his recent autobiography Eat and Run of chasing the "zone". He describes it as: "That instant when we think we can't go on but do go on. We all know the way that moment feels, how rarely it occurs, and the pain we have to endure to grab it back again.

"I'm convinced a lot of people run ultramarathons for the same reason they take mood-altering drugs. The longer and farther I ran, the more I realised that what I was often chasing was a state of mind – a place where worries that seemed monumental melted away, where the beauty and timelessness of the universe, of the present moment, came into sharp focus."

Ian Couch, the owner of Adventure Hub, which organises the Norfolk Coastal Ultra, says running an ultra is about self-discovery.

"When you realise what you are capable of and what you can put up with, it is very liberating," he says. "You're used to being clean and warm and dry, and when you're out in the elements for the length of time it takes to run an ultra, you get this voice in your head telling you to stop … and if you don't, then that's very rewarding. There is an addictive element - but I think it is more mental than physical."

Believe it or not, running 100km was fun. Whatever caused my ultra-runner's high, I'm busy plotting how to experience it again.