I did only one marathon a year, and that was London in the April. 3/4 months build up over the winter & spring, with a job, was the max mental & physical effort I was willing to put myself through. Now that I am older, I am kicking myself that I did not do Berlin in the September but, you know what, I could not be arsed going through that build up again from June. That's a failing of mine which irks me sometimes.... so I did 10 km's and the like over the summer.
So what I am saying is, if you only get 1 chance to get it right then you need to approach the marathon like you really mean it. It's too hard to go in half hearted. Get planning; I found Pfitzinger's ' Advanced Marathoning ' an excellent resource. It answers just about any question that will come into your head about marathoning. You will see the answers to most of your failings right in this book.
I will guess your thighs gave out as you had not exposed them to race relevant stresses whilst training. More threshold efforts & long runs would callous you to the demands of the race.