I would assume that they are conditioned and trained (odd, you wouldn't think you need special training to run, but in fact most people do if they want to minimize the impact) which is going to be the hard part for most people, if you're not used to running, the impact on the joints and such is high (higher, remember, we doing it to lose weight, so your packing on extra pounds into that impact) and if you have not been doing a lot of it, your joints and such are not used to that kind of repetitive impact and it can hurt a lot of people once they start getting to middle age and their bodies ability to recuperate and heal begins to slow. (Run-on sentence winner of the day)
For most people starting out on a physical program - because they are overweight, implying they have not been as active as they might have been - its better to start out with something you can do without too much pain, and that you can stick with. To start out with something hard and hurting - and if you're not use to running, its going to hurt for a long time, makes it very difficult to do the real thing, which is not going out running a couple of times, but a sustained program of ongoing physical activity. The real need is to build a program that will work and that you can stick to in the long run and build on. I'll bet that 'running to lose weight' is the most often started, and most often stopped program out there. A couple of days ain't going to do it, but building a life that includes a lot of that, will.
To that end, the most successful program, is one you share with someone, a gym buddy, or running buddy, someone who can make you do it, even when you don't want to. And 'not wanting to' was was got you there in the first place.
