dbail2,
I did the trail ride 2 years ago. Yes, you do need reservations, but I don't think they book up too quickly. The number to call is 407-WDW-PLAY. The cost is $32.00, and the minimum age is 9.
The trail ride itself is very tame. The horses are great- they sort of size you up when you get there, ask if you have any riding ability at all (I do, although you don't need any!), and match you to your horse. The ride lasts about 45 minutes, and it basically takes you back through the woods at the Fort Wilderness. Like I said, it's very tame- no trotting or running at all. It's at a nice, slow pace. There's really not all that much too look at on the trail outside of the wildlife, but we enjoyed it. Strike up a conversation with one of the guides, if possible. They were excellent.
Let me know if you have any other questions. Have fun!
Mega