You see all shapes and sizes at parkrun. The quickest ones do tend to be skinny though. I think running helps keep my weight under control.
You can mix running and walking to build up. That's what the Couch to 5k programme does. Running for long periods is hard. Got to get the heart and lungs working to build fitness.
I admire those who can run a marathon. I just don't feel up to that yet. Maybe I should enter one and then I'd have to train for it.