Smug is asking a cycling group about this... but generally, people are different and some will have issues, innate or otherwise, that make being active challenging, so group-level prescriptions always sound a bit harsh, and of course they're generalizations.
A GP friend tells me there's a broad split in her patients, the unhealthy and getting unhealthier, and the 'professionally' healthy (people like me with their gym memberships). It's most defined by socioeconomic background, of course, but younger people are getting less healthy than their peers, especially in the poorer groups. Younger people apparently don't want to be told that they're on a course for chronic ill-health either. It correlates with driving and jobs that rarely require any physical activity, a significant number of people do zero exercise. There's also a cohort of middle-class worried, who are ostensibly in reasonable physical health, but turn up with diagnoses they want confirming. Anyway, once upon a time, she would have prescribed words for all these. Go do something active, get outside, find a hobby. These days, no one wants to hear that. Anyway, like many GPs she's counting down the days to a patient-free early retirement.
I got heavy after a significant road accident, which meant – as I lived in the US – I spent years effectively driving between massive portion sizes. Sometimes I had to walk across a parking lot, though I tried to avoid it. Finally had enough of getting breathless at the thought of a walk and my body continuing to wobble for five minutes after I'd walked down the stairs. No magic formula, cut down on the crap food (but no actual diet) and start regularly exercising. These days I do a good hour of proper aerobic exercise every day (swim most days, hike at the weekend), but I balance that by eating and drinking well, I'm not living off lettuce.
I have a niece who won't walk the five-or-so minutes to see her grandmother, she has to drive. She claims walking makes her legs hurt. She's 21 and other than her weight in good health.