"That, if you don't mind me saying it, is the perfect example of shit posture," Major Martin Colclough says warmly. In a chilly shed at the Army Development and Selection Centre (ADSC) outside Woking, I am attempting to perform as many press-ups as I can in muster in two minutes. Somewhere up above me Sergeant Claire King of the Army Physical Training Corps (APTC) is holding the stopwatch, and Major Colclough, Sophie, the PR, and Linda, the Guardian photographer, are all looking on, smirking. Down here on the ground, the situation is less amusing: I am painfully aware of the smell of the carpet and the sound of my breath and, out of the corner of my eye, the gleam of Sgt King's shiny new boots. Sweet Jesus, I think, when will this hell end?

The ADSC is the starting point for civilians who want to become soldiers. More than 10,000 people apply each year, at five such centres across the country, with around a third of applicants coming to this specific centre. The first phase of the selection process takes two days and includes aptitude tests, fitness tests, medical checks and a formal interview. The results will reveal which jobs, if any, the applicants are appropriate for, and allow them to enter the second phase of their application.

The entrance level fitness test, which I must perform today, involves a series of strength tests and a mile-and-a-half run. Around 90% of applicants pass the first test. The vast majority are male - just 8% of those serving in the army are women, and last year, of the 670 newly trained officers, 90 were female. Women generally fare less well in the fitness test, a fact that Maj Colclough explains as "just a physiological artefact of being a female. Most women have lower levels of aerobic fitness, smaller lung capacity, a smaller heart, they're smaller in stature, with a smaller muscle mass, and so a smaller percentage will qualify."

In truth, recruits are not expected to perform the press-ups (or the even more dispiriting sit-ups that follow) until 16 weeks later, after they have completed a training programme. At that point, they are tested again to see if they are fit enough to join the army properly - this includes not only a repeat of the first test, but also a six mile march in an hour and a half, carrying a load between 15 and 25kg, and a 'single lift' test, raising a weight between 25 and 45kg up to a height of 1.45 metres (representative of the height of the back of an army lorry). The army fitness programme in tomorrow's paper is based on what happens during those 14 weeks and, by the end of that time, you should be able to do more than the 20 shambolic press-ups I managed.

After the press-up humiliation comes the run. This takes place outdoors, and is normally devised to incorporate a variety of terrains, but today we are running around a track. A mile and a half means six laps of the track with a half-mile warm-up. I am looking forward to the run. Running is what I like doing. And while I will confess I have been a little slack on the morning run front since the colder weather arrived, I'm fairly confident I will pass this bit. The only hindrance is that I am attempting it with a bad chest infection. In fact, readers, if I have learned anything from this whole army fitness malarky it is this: never go for a two-mile run with a chest infection. Not only will you run incredibly feebly, but you will also make yourself really ill and have to take time off work and spend your days at home coughing in the manner of a retired coal miner.

But I digress. Out on the track, Sgt King joins me for the warm-up, and as we run she tells me about the nine years she has spent in the army, first in the military police and then in the APTC. She tells me about the jobs women can do in the army, which includes everything bar serving in the infantry and the armoured corps; how though women cannot fight on the frontline they can serve in frontline units as a medic, for instance; and how there is yet to be a woman strong enough to join the paratroop regiment. She talks about growing up in Brighton and how much she loves running and about the iron man triathlon she will be performing later this year. She is strong and lithe and cheery, while I sag along beside her, forlorn and breathless.

Back at the start again, I commence my test run alone. Round and round the track I wheeze. It is a crisp winter morning, and I alternate between running through the shuddering cold wind and the glare of brilliant sunshine. In the distance I can see trainees in their fatigues stamping through the mud, and from somewhere rises the faint chant of "left-right, left-right, left-right." The ADSC is set in the middle of the Surrey countryside, and as I chug around the circuit I try to distract myself from the pain in my chest by looking at the cluster of low-lying buildings that make up the centre, and the spindly tops of the bare trees beyond. When this fails, I try to mentally chant some Puff Daddy lyrics: "I am a mountain/I am a tall tree/I am a swift wind sweeping the country." But this only serves to remind me that I am no such thing: I am a weedy journalist galumphing around a running track in a peach T-shirt.

Whenever I pass Maj Colclough and Sgt King and Sophie and Linda they bellow encouragements. "You're doing well!" shouts Maj Colclough. "You're halfway through now!" If I had any breath to spare I would tell him to shut up. The official army standard requires applicants to complete the distance in 14 minutes. I lollop in a minute late. "That cut-off point," Maj Colclough explains as I flail about for air, "is based on research that links aerobic fitness to an individual's predisposition to musculo-skeletal overuse injuries during training. By being more aerobically fit you can significantly reduce your risk of injury. Those who complete the run in more than 14 minutes are two and a half times more at risk of injury than those that get 10 to 12 minutes. So it's really about us being a responsible employer."

The nice thing about the strength tests is that if you are really rubbish at one of them it does not necessarily mean you fail. There are five tests in all, the results of which are fed into a computer, along with your height and weight, and the result is calculated. Strength in one particular area may denote that you are suited to a particular kind of job, such as building bridges in the Royal Engineers, for example. I am not terribly optimistic about the strength tests. Not only am I now coughing uncontrollably, but I am not a tremendously brawny person and on the whole journalism does not require a great deal of brute force. The tests that lie before me require the use of several strange-looking machines, which Maj Colclough explains will measure my attempts to lift "immovable objects attached to a load cell, while the amount of force you apply to the load cell is recorded to a digital readout in kilograms of force."

First up is heaves, where I must stand on a chair, cling to a bar above my head and attempt to hoik myself up to meet it. This I cannot do, but Sgt King reassures me that nearly all women fail this test. The tests of back extension, static lift and dynamic lift strength all necessitate standing on pieces of equipment, striking an ungainly pose while pulling or pushing weights, and holding it for the count of three. I am surprisingly much better at it than I anticipate, and on the back extension and static lift, find that I am capable of lifting weights greater than my own body weight.

Finally, we head outside for jerry can carrying. One does not often have cause to carry jerry cans in real life, so let me tell you they are heavy little beasts and within approximately five seconds they hurt like blazes. The objective is to see how far you can carry the cans without putting them down, up to a distance of 180 metres. "Women tend to find this task quite difficult," Maj Colclough tells me. "They can pick them up easily enough, but the problem is their grip strength, which means they have difficulty continuing to hold them." I manage 90 metres, which is, Sgt King informs me, rather good.

Buoyed by my strength test success, we head back inside to compute the results. I find that if I could just prune back my run time, I would be fit enough to train for 16 possible army jobs, including combat medical technician, nurse, radiographer, musician, chef, driver and - thrillingly - dog trainer. And while I am unlikely to give up my desk job for a life on the frontline, it is an inspiring thought that just 16 weeks from now, I might even be able to do some decent press-ups.

· Follow the army's 16-week training programme, free with the Guardian tomorrow