Having both walked and run marathons and trained thousands of runners and hundreds of walkers, I know that completion is the most important goal and that walking a marathon is both easier and harder than running a marathon. The amount of distance training per week is somewhat less than for running and the intensity level is quite a bit less. But, as a marathon walker, you will be on your feet and moving for a much longer time than the typical marathon runner. The bottom line is that training is necessary and needs to fulfill the basic and specific needs for the marathon distance. Marathoners come in all shapes and sizes.
Some marathoners are stick people, but most are just folks and some are decidedly overweight.
Marathoners come in all ages. I've trained marathoners from their late teens through late 60's. No one is too old to start a fitness program and from fitness to marathoning just takes some common sense. I've trained asthmatics and athletes with both type I and type II diabetes to successfully complete the marathon. I actively discourage children from participating in marathon events.