I never thought I would need to know so much about physics to become a runner. But it turns out that sports science is a real thing, not just a degree people take when they don’t want to leave education but only really like their friends and aerobics.

Nick Anderson is a Team GB and England athletics coach. He has agreed to show me how to trail run, ie swap the pavement for the park. “Top runners would do both, on and off road, because they complement each other,” he says. It is basically grass versus tarmac, although as soon as you are on grass, you are also on mud, twigs, stones and tiny, vicious hillocks.

Trail running is the most natural form of exercise and therefore the most inspiring, but this is according to a man who loves to run. If you hate to be breathless, you would swap all the scenery in the world just for it to be less hard. There is a downside: the energy return on soft surfaces is less (imagine a ball bouncing; it would go higher the harder the surface. That ball, that is your body). So, it is easier on your joints, but makes it harder to shift yourself.

You also change the way you land your feet, because you are more keenly aware of the ground below you; there’s less chance of getting injured. Later, I overhear a dispiriting conversation between Nick and another runner, where they conclude that you will always end up with an injury if you run enough. “Your biomechanics will find you out,” he says ominously, as if they are the Stasi.

Trail-running shoes are different, although not so different that Saucony couldn’t design a road/trail hybrid, which is what I wear. The main change is in the side support: there is more lateral movement on uneven surfaces and you want to protect your foot while staying as light as possible. The foot is bound in my new trainers and there is a more aggressive tread pattern, so you can collect more mud and keep it in your house.

And whaddayaknow, the science is all true: running on grass is harder, running on an upwards gradient harder still. Running downhill is counterintuitive: you need to lean forwards, run light, with an agile, pitter-patter foot plant; relax and be loose; don’t dig your heels in; let gravity pull you down the hill. Putting the brakes on causes injuries; watch for rocks, gravel, scree, loose surfaces and roots. There is a lot to think about.

Running off road is more romantic: you are sacrificing speed, so you become less fixed on numbers: a bit more Corbyn, a bit less Blair. Also, wildlife. You won’t meet a pigeon on the A3.

What I learned A beginner should do 50% of their training off road, since it builds fitness faster and cuts through the corrosive boredom