Exercise on "prescription" is not a new idea, I have been able to refer patients for 3 months free membership off the local gym/pool for years. Take up is good but unfortunately not from the people who most need to lose weight. Therein lies the real challenge of motivating those who most in need. The people who are a little overweight know that 6 weeks work will have a big impact, people who are significantly obese are looking at months and months of work and changing their way of life.
Back when I lived in the UK, I was given a prescription for exercise. I went to the local Gym, and got 3 months membership, and a training plan and a tour of all the machines. I don't think I lasted the 3 months. Being a fat woman in the Gym is not a pleasant experience. The Gym I was sent to was also noisy, and bright, and sensory overload. The gym membership was free, but the sports bra I needed to be able to do any exercise without being in total agony was £50.
When I injured my feet and was sent to a podiatrist,with the referral from my GP that I had gone from regularly walking 20+km, to being unable to walk 2km due to foot pain. The podiatrist said to me "Your weight isn't helping, have you considered walking for weight loss?" FFS.
Even when I was doing 1000+km a month, with weight slowly disappearing doctors would still assume fat == no exercise. I got told by one doc to cycle more. When I went back and said I'd done just over 2000km since our last appointment, but that wasn't as much as the 2400 I had done in the same time before the previous appointment and I was worried I couldn't fit any more cycling in, his face was wonderful...
I don't know what the solution is to the obesity problems of our society. But I know that many people are going out of their way to make it a damn site harder than it needs to be...
That said, cycling to work can be a really effective measure. If you're work is 7.5km away, that's 15km per day, which is 375kcal of exercise. If you don't change your diet and were already eating enough to maintain your weight, that will lead to a weight loss of about 200g a week. Not fast, but steady it adds up... it's 10kg over a year.
But cycling to work requires infrastructure... and a waterproof jacket... and a reliable bike... and a commute that is far enough to be good exercise, but not too far away...
J