Good news: fitness snacking is the key to getting into shape, according to Samantha Cameron’s personal trainer, Matt Roberts.

The less good news is that there are no actual snacks involved – sadly it doesn’t mean going for a walk with a packet of crisps. Fitness snacking is doing short periods of exercise throughout the day. Ideal for sedentary but busy people with little time for spinning classes or swimming.

Taking the stairs instead of the lift. Photograph: Graeme Robertson/The Guardian

Roberts is busy with clients – probably Sam Cam or Naomi Campbell, who he has also worked with – he will call back later. In the meantime, I devise my own programme. How hard can it be? I cycle to work, that’s good, even if London’s diesel fumes probably undo any benefits. And I take the stairs – three storeys – to get my mid-morning coffee. At lunchtime, which I would normally spend at my desk, I take a brisk walk outdoors, along the canal.

When are you supposed to eat in all this? Well, cleverly, I have brought an actual snack along, to have on the go. Turns out fitness snacking does mean going for a walk with a packet of crisps, after all …

Roberts calls back. He is not as impressed as I had hoped: the cycling is good, but for the stairs, while obviously better than the lift, he only awards half a point, if I took them at speed. (Points are good, they will help me achieve my fitness goals; I am after 10 a day.)

As for the Walkers walk, that gets nul points. “We know that if you do short blasts of high intensity, there is a positive shock to the system, you get as much benefit – arguably more – than going for long durations.”

One of my daily “snacks” needs to be this kind of high-intensity activity, a five-pointer, even if it is only 10 to 15 minutes long. He suggests a run, maybe just 2km, but really going for it. “You want to get hot and sweaty and out of breath.”

He tells me about PHA – peripheral heart action – doing an arm thing, then a leg thing, then an arm thing, etc: “A squat, followed by a press up, followed by a lunge, a pull-up on a bar – arms, legs, arms, legs – and that’s a great way to get a workout done.”

A brisk walk – with crisps. Photograph: Graeme Robertson/The Guardian

Then he is on to my abs, setting me sit-ups – 150 of them. “You’re going to feel as though you’ve extended you heart capacity, like your lungs have grown.”

It sounds brilliant. It is also clear that I am going to have to come to work with some kit tomorrow. And a whole new attitude. Step aside Sam Cam, Naomi Cam. Make way for the fitter, new, toned Sam Woll.