"It felt like we were marching through a battlefield. People were gasping for breath and shouting for help. Lots didn't make it to the end, either collapsing from exhaustion or unable to take the pain any longer."

That is how one friend dramatically described the 24-hour, 100km charity walk from London to Brighton. What was I thinking when I signed up? But since making that rash decision last November, I've had little choice but to lace up my first ever pair of walking boots and take to the streets of London. It has been a refreshing – and blistering – experience.

A few years ago, walking was a hot topic, with charities and media commentators encouraging us to get a pedometer and walk 10,000 steps a day. But it seems we didn't listen: according to the historian Joe Moran, walking has declined by 25% in the past quarter of a century in the UK; a YouGov poll recently revealed that as many as a quarter of British adults walk for an hour or less a week – and that includes getting to the door of your car. Two-thirds of adults in the UK don't do nearly enough physical activity. The result is clear: sedentary Britain is facing a public health crisis.

At a recent TED lecture, the author Nilofer Merchant said sitting is the "new smoking of our generation". The phrase has been picked up by public health academics and experts, who warn of a worldwide pandemic of inactivity. Even going to the gym in the evening isn't enough to offset nine hours of sitting still in the office, according to studies. Walking needs to be part of everyday life – your commute to work, your journey home, your visit to the shops, your lunch break, and even the way you work.

Merchant has called for meetings to take place out of the boardroom and on foot, not only because it is healthier, but also because it encourages conversation and productivity. Aristotle would be pleased – apparently he loved to walk and talk, and would wander the Lyceum in Athens as he lectured to his followers. Sigmund Freud and Charles Dickens were also known to work on the move.

Perhaps the importance of combating our desire to sit still is spreading: estate agents in part of the US and Canada are starting to market homes according to their "walkability" rating. That is a measure of just how many places (pub, shops, post box, cinema, schools, offices and so on) you can reach by foot from your home. My home – in Finsbury Park, north London – is a "walker's paradise", according to the website walkscore.com. It makes sense. My surrounding streets are close together, with plenty of corners to bump into someone for a chat. If you head out to the countryside your walkscore goes down, because it's harder to cope without a car. People who live in walkable neighbourhoods are not only healthier and happier, but also 6-10lb (2.7-4.5kg) lighter, says Walkscore.

So although the idea of walking non-stop for 24 hours fills me with dread, it has propelled me on to the streets, forcing me to walk hundreds of miles along the pavements of London. What an eye-opener. In the last few months I've watched buildings being knocked down and built up, and witnessed things I would never have seen from the darkest depths of the tube. I've seen an entire theatre being built, and on walking past one evening to see its doors open to visitors, I decided to become a volunteer there. Walking has created a whole new connection with my community and provided me with hours of pleasing conversation with my walk-to-work buddy.

There are signs, too, that the decline in walking may be slowing or even reversing. Last week the UK Chartered Society of Physiotherapists said that despite the overall decline in activity, more and more people are ditching the train, bus, car or tube on their way to work. Over the past decade the number of "walking commuters" has risen by 9%. And it's not just walking to work that's becoming more popular. There's a walking book group, walk speed-dating events and walking therapy sessions. There's even a walking scheme being trialled in Berkshire which logs how far people walk using a system similar to the Transport for London Oyster card.

By this time on Saturday, I will hopefully be several miles into my epic 64-mile walk. I know there will be moments when it will seem far easier to sit down and stop. Maybe I'll be one of those people who described it as a battlefield. But whatever happens this weekend, the real challenge will be to keep on walking – not just to Brighton, but beyond. With a name like mine, I have no choice.