Gains in fitness come from consistent workouts. Workouts on numerous days of each week carefully mapped out to allow for stress followed by quick recovery and then more stress. A marathon, however, throws a bunch of inconsistency into the system. You train and train. You get in your consistent workouts for months and months. Then, you purposely drop the consistency. You taper. You relax to let your body get good and recovered, ready for a personal best. Then you go out and run all out for hours. You introduce a huge stress onto your system from which you cannot as quickly recover. So, you have more inconsistency as you allow you body to recover from the race effort. When all is said and done, several weeks after the marathon your overall fitness is not as high as it was several weeks before the marathon!
On the other hand, it your fitness is WAY above where it was when you got off your coach and started training for that marathon in the first place. The frustration for me is that I am greedy. A huge marathon PR is not enough. I want to milk that training into a bunch of PRs at a range of distances! I want to recover in a few days and then go out and set more PRs 1 or 2 weeks later! It has worked that way for me before, but that was partially because all those other PRs were low enough to knock off with sub optimal races. After a 3 years bout of doing marathons followed by other PR attempts, none of those other PRs are looking easily to go get. Besides, now it is summer and the weather is going to be pretty warm for personal best results.
I guess what I am saying, and it is actually coming to me only as I write this, is that maybe I should be cautious as I rebuild from this marathon. Especially, after experiencing some twinginess in my hamstring this morning. Not during my 10 mile run, but later just walking around at church. This is the same area that has remained occasionally achy for more than a year.
Looking back at my original premise though let me offer an obvious huge rebuttal. I have found, that if I do not have a goal event to train for, or if that goal race is not long enough, I will not be consistent enough in training. My fitness gains are low because I do not fear the event. The fear of a marathon forces you to put in consistent training which does lead to gains in fitness even after you account for losses due to the taper, the race itself, and the recovery.