ATHLETICS: ON WEDNESDAY I was sitting at a fancy teahouse in Boulder, sipping on some Organic Moroccan Mint in the hope it might ease my mild altitude sickness, and talking with a former college team-mate about whether we should enter one of the three Thanksgiving Day road races organised for the next day, writes Ian O'Riordan

It took a while before we noticed two middle-aged men at the table next to us, one wearing navy running tights and a white T-shirt, the other running shorts and a yellow vest. They'd walked in a while after us without generating even a passing glance. Boulder must be the only town in the world where you can walk into any café bar or restaurant wearing a pair of running tights and a T-shirt and comfortably fit in.

Back in 1991, Runner's World magazine voted it the Running Capital of the US, and for good reason. Since the 1970s runners have flocked to this small Colorado town searching for that most promising of training aids: high altitude. At 5,433 feet - more or less a mile above sea level - Boulder won't necessarily take your breath away before you even start to run. (Although the scenery certainly will.) But what Boulder also has to offer - along with its laid-back lifestyle and near-perfect running climate - is easy and open access to the Colorado Front Range, the several-hundred-mile stretch of the Rocky Mountains from Mount Evans to Longs Peak.

Run just a mile or two out of town up into the mountains and the altitude can very quickly be doubled. Keep going a little further north, into Rocky Mountain National Park, and the air thins out considerably more again. At Longs Peak, which rests at 14,255 feet, the air practically runs out.

Earlier in the day, we'd run in warm sunshine up one of the dozens of well-marked trails that take you high into the mountains, which suddenly turn very remote. The start of each trail is marked by a sign rating its level of difficulty, ranging from the moderate to the strenuous to the impossible.

Flagstaff Mountain trail offers some of the best and dramatic views for the least amount of effort, and although it was very unfairly rated as moderate, it was beautiful beyond words.

Something about this landscape also feels subconsciously familiar. This is big sky country. This is Indian country. This is John Denver country.

It's also familiar from the old television add for Kellogg's Start, which showed Steve Cram running up the trails in Boulder, rewarding himself at the top with a large bowl of cereal and fresh, creamy milk.

The air up here is hugely contradictory. Each breath fills the lungs with a burst of freshness that at first feels superbly rewarding, but then feels as if it's not quite doing its job, that the air isn't quite getting in. This, of course, is the whole purpose of altitude training, to run where the oxygen level in the air is far lower than at sea level, therefore forcing the body to adapt by increasing the oxygen-carrying red blood cells.

This, however, doesn't happen overnight, and it takes at least three weeks for the benefits of altitude training to kick in. But when they do, and provided the timing is right, then running back down at sea level feels significantly easier. Altitude training was one of the first truly natural performance-enhancing aids, and probably the last one too.

Still, it took a while to catch on, and it took the emergence of the east African runners used to training up in the Rift Valley and other high-altitude areas to lure the runners from America and Europe to consider doing the same. Back in 1972, America's best marathon runner, Frank Shorter, came to Boulder to train for the Munich Olympics, and the high altitude clearly helped as he won the gold medal.

Shorter still lives in Boulder, and his image is the centrepiece of a large mural on the way into town showing a group of runners emerging from the mountains.

Boulder is a changed place from the 1970s, and while it can't seem to make its mind up between being a hippie town or a yuppie town, it definitely oozes wealth. If it's one of the best places to live in the US, then inevitably it must also be one of the most expensive. The population of 90,000 is not growing very fast.

We didn't pass a soul on our run (or, thankfully, any of the bears or mountain lion common to the area), but that's not to say there weren't lots of other runners around. Boulder attracts runners of every level and every age, and even if they aren't all Olympic quality, they all train like they are. There are at least 10 running stores in town, and in the one we visited, Boulder Running Company, the man behind the counter was Mark Platjees, winner of the marathon at the 1993 World Championships.

The place was also incredibly busy, so we just picked up a couple of "Sea Level Is For Sissies" T-shirts and left.

A small enclave of Irish runners have also been lured to Boulder over the years, and, until recently, Keith Kelly was living in the basement of former world marathon record-holder Steve Jones' house. Kelly remains one of Ireland's best running talents of the past 20 years, and while he's now based in Boston, he appears to have left quite an impression on the place.

Kelly may not have won Olympic gold, but he did win a couple of Boulder's road races, and as far as local reputations go that was just as good.

Because all runners in Boulder take themselves very seriously. Just like the town's second favourite sport, cycling, image is everything, and if you don't look the part, then you better not take part.

Based on various measures of health and disease, Colorado is ranked as America's fittest state, so on that basis, Boulder is definitely America's fittest town. The evidence is staring you in the face. Here, heart-rates and blood pressure are more important than interest rates and peer pressure, and if you are carrying any excess weight then you better only come out at night.

There are several groups of Kenyan athletes based here too, but they don't look any fitter or healthier than the locals, when of course anywhere else in the country they would stand out as the leanest of the species. Boulder is a clear example of how your standard of health defines your standard of living, not the other way around, and also how good health, in a way, can be the most infectious disease of all.

That's normally a good thing, but out here it means running a local road race is also a hugely competitive affair, where every runner next to you not only wants to look better than you, but beat you too. In any other town you can always fake your fitness and jump in just for fun. But not here. That, on top of the high altitude, was enough to convince us to pass on the Thanksgiving Day road race, and go snowboarding instead.

"It takes at least three weeks for the benefits of altitude training to kick in. But when they do, and provided the timing is right, then running at sea level feels significantly easier. Altitude training was one of the first truly natural performance enhancing aids, and probably the last one