Yes, you can do it withou running anything over 20 miles in training.... I ran no more than 32 miles a week, trained for 18 weeks (was already running halves) and adding 2 x spin classes and 2x strength classes. I would have run more, but I had plantar faciitis in both feet, so less time on my feet was where I had to compromise. I did the MK marathon, in atrocious weather, didn't stop once, and was only 10 mins outside my dream time - which was pretty good considering the course ended up half mile long due to re-routing for flooding. It was a hellish 26 miles in places as was so wet and cold and so many floods on pathways meant lots of picking your way through the mud instead - and I must admit I found the last 6 miles a totally different story from the first 20 miles.
What would I have done differently? Well, had I not had the foot issues I would have run at least once a week more, adding at least another 6 miles to weekly milage. I wouldn't have done more than one 20 miler but might have done several 17/18 milers instead of 1.
The schedules are good as you can see what you've achieved and it makes sure you get your lower milage every 4 wekes as a mini recovery. If I can do it, anyone can... its just consistent training for a good few months, and that'll get you though.