So it is possible to tour the parks and do the races. Even all the races that weekend. It can also lead to attempting crazier things in running that you once believed were impossible for you.
I didn't hit the parks after my WDW 10k, but I've been to the parks the day of every other race. Having done it twice now, it is tradition for me to go to AK after the race to get photopass pictures with my medal(s). (See my avatar for an example.)
I have noticed that, for a 10k/half challenge, I'm generally more wiped out the day of the 10k than the half. Not sure quite why.
As for before/after - I tend to end up flying home on Monday after the race, so I get a bit of parks in after but not lots. As I'm flying cross-country, I fly in early to give me some slack time if something goes wrong. And as long as I'm there, I might as well hit the parks...
Plans for my next two trips:
Princess: fly in Thursday (possibly expo if I get in early enough), park/expo Friday, park Saturday, race and parks (probably AK) Sunday, park and fly home Monday. Exact parks aren't planned yet, will need to consult with my daughter as we get closer.
Star Wars: fly in Friday with intent to hit expo, park Saturday, park (probably AK) Sunday, park (probably Studios) and fly home Monday. I may see about adding a day and flying home Tuesday. (Friday is an AP blackout day, so I can't add a day to the front.)