We did this last year, and had a lot of fun despite the weather. We're not planning on doing it this year because, well, I've been in Disney June 12th before and I certainly wasn't in the mood to be running, even late at night!
The race itself was fun - you basically start out in the parking lot, go into the park and backstage for a bit by Africa/Rafiki's Planet Watch, go through part of Asia and Dinoland, and then finish the 5k back out in the parking lot. If you're in a pair, you have to wait at this point until your partner also crosses the 5k finish. You will be assigned a corral (last year there were 13 total - the first few were for female teams/male teams/co-ed teams that could prove fast times (?) - and the rest of the corrals were for all the other co-ed teams and were assigned based on how early you signed up for the race, I believe.) Every 5 minutes, a new corral is released to go over the starting line. We were in corral 9, so we had to wait 45 minutes to go - which meant there were people that were LONG done with the 5k by the time we even started! (On the bright side, if you're in one of the earlier corrals, you barely have to worry at all about the pacing requirements!)
After you finish the 5k, you then go further into the parking lot and do an obstacle course that consists of a balance beam, a cargo net you climb over, a cargo net you crawl under, and jump over/climb under some hurdles. (Note: you do NOT need to do the obstacles if you are uncomfortable with any/all of them. They would at least like you to walk alongside your partner while he/she is doing them, and give it your best for the ones you're okay with - like ducking under the hurdles instead of climbing over them/etc)
After the obstacle course (which you again have to wait for your partner to finish, but this is much easier than the 5k due to the much shorter distance), you get handed a scavenger hunt booklet and a pen (last year it had a light on it so you could see in the dark) and you basically end up re-tracing your steps through the Animal Kingdom park to answer some fun clues, and cross the finish line in Dinoland and get your medal.
...or at least, that's whats supposed to happen. Last year was a bit of a debacle because the race got cancelled halfway through, and people were at various stages of the course when it happened.
As for the after party, they have a lot of the "non-animal" based rides open (Dinosaur, Everest, It's Tough to be a bug, a lot of the other rides in dinoland, kali river rapids...maybe a few others) and they have a DJ playing music to dance to. Some of the shops are open, and various food places are open (they'll give you a bag with a banana, a muffin, and maybe some other goodies, as well as water and/or powerade when you cross the finish, but otherwise you have to pay for food/beverage)
Overall, the race was a TON of fun, if you could keep an open mind about the weather. My honest recommendation is to pack a change of shoes and socks or
crocs for JUST IN CASE into the bag you check at the beginning of the race - after we were done running through water up to our knees, we were DYING to get out of our sneakers and socks, and there were no shoes to be bought in the whole park!
And especially in June, make sure you hydrate a lot!!!!! But you'll have a ton of fun - especially when you cross the finish and get the medal - and disney is great about making their races a fantastic experience!
We're going to drive down on Friday and make it a girls weekend. Signed up and hotel is booked!! I'm counting the days already!!!! It's been awhile since this mom has been on vacay by herself 
I just did the Princess 1/2 and had such a great time that I couldn't wait to sign up for another Disney race. It can get $$ so I'll probably stick to 2 Disney races a year. Good luck everyone! 
