Ok, the bikes are past their best, but mine rode fine. I arrived just before the gates opened. Even though it was Easter Sunday, there were only a few other cars entering at that time and it was obviously the coolest part of the day. I booked a bike in advance, which might not be necessary if you go that early, but certainly is if you go later - by the time I returned at 11:30, there was a waiting list for bikes and people just had to wait for bikes to be returned before setting off.
I went really slowly for the first half, because there was just so much to see! I was stopping every 30 seconds or so. A lot of people were flying past me and would have missed a lot. I don't really see the point of doing the bike ride if you go fast. May as well not get hot and sweaty and do the tram instead. At the observation tower I turned around and went back the same way, as I heard that there was less wildlife on the second half of the loop. On the return leg I pretty much zoomed back in about 40 mins as I had seen so much on the first leg and it was getting very hot. I did still stop a few times and saw some new things. I saw so many people just starting off at the hottest time of the day and it seemed a bit crazy - plan better!
I liked the bike option because I could take things at my own pace, which allowed me to stop often, but I was also able to go all the way to the observation tower and back within 3 hours. I'm glad I did as I saw different things the further along I went, although for those with less time/physical ability, even going 2 miles out and back would still be worthwhile. Definitely do-able on foot, but it would be a long day in the sun to go all the way to the tower and back.
I went on a 1.5 hour airboat tour after, which was super fun and interesting, but I saw many more gators and birds at Shark Valley. It was a great combination to do for a daytrip.