I keep meaning to reply to this thread but withdrawing my response, because it's too simplistic to answer the original question with just a "yes" or "no".
Last weekend I ran the London Marathon for the first time, after achieving GFA at my first marathon in Manchester the year before.
I don't think I'm alone in saying that I never thought I'd run a marathon myself until last year. The reason was because I never had to try, therefore I didn't think I could. My running partner, who I usually do my long runs with, was the one who spurred me on to do my first marathon at Manchester. With the bigger races, a 12-16 week training plan for each level is provided.
Yes, the months of training is intense and you need to be dedicated to doing at least 4 runs a week including a long run, and maybe one or two speed workouts if you're aiming for a time. All this is preparation so that your body can handle the demands of the marathon on the day.
I say if you are able to cope with training in all weathers, sometimes getting up at 5:30 in order to fit a run in before work, then the marathon itself will feel a lot less intimidating.
The cut off time is generous, and there are participants of all shapes and sizes. Don't write yourself off yet op, if you haven't tried.