You don't have to do a lot of exercise to lose weight, you just have to not eat the calories back. And it's tempting to, as exercise can make you bloody hungry.
If you run for an hour a day, even very slowly, so say six miles, that's a 600-ish calorie burn right there. If you're eating 1500 a day then your net calories will be around 900 and you'll lose weight (usually).
There's more to it though. You generally don't see fat distance runners, not just because they burn a lot of calories training, but because they don't eat shite that affects their training.
When I was last in serious training, before I was pregnant this time, I didn't drink much booze and avoided takeaways etc - because I knew I'd feel awful on my morning run if I had curry and wine churning around inside me. And when weekends are taken up with races, you're not trudging around a shopping centre being tempted by giant creamy coffees.
Also, running is easier when you're lighter. If you're a proper runner, there's an incentive to lose weight because you can then run quicker, more easily. I took 3 mins off my 10k time when I lost a stone and a half.
Being surrounded by slim, fit, fellow runners helps too. You don't want to be the fatty slogging along at the back of the pack with chafing thighs. So you do something about it.
So yes, it's not the exercise alone that makes you lose weight (though that's undoubtedly the main part of it), it's the whole lifestyle change that comes with running.
Put simply, you see more fit people at a local fell race than you do in a cinema or shopping centre, so "fit" becomes your new normal.
And on the other hand, if you spend your weekends at soft play or in the shops or whatever, you'll be more likely to think fat is normal, because the slim and fit people are generally off doing their thing.