Fantasy Island is a trail system built for mountain bikers right in Tucson. It's on State Trust Land, so you have to get at permit to ride. Note, there is one guy who goes around giving tickets. So depending on your perspective, you either have a good chance of being ticketed, or a poor chance. However, while you can buy a permit in Tucson, the state office is not open on holidays or weekends. No other public recreation facilities (such as state or national parks sell the permits).
Fantasy Island is interesting. The first part of the trail zips back and forth over washes. Generally you can take the washes at speed and the momentum you get coasting down will get you about 3/4 the way up the far side. Watch out, however, for washboarding in the trail, and large, loose rocks. The curves are ramped, so you can take some of them at speed. Watch out, however, because you'll run off the trail and into the cactus if you go too fast.
My GPS said it was 8 miles to the bottom of the Bunny Loop. The whole ride I did was between 11 and 12 miles long in an hour and half, plus stopping to take pictures. The twists and turns makes all but the Bunny Loop highly confusing to navigate. The trails are well marked in some places, and poorly marked in others. I would have spent more time at FI, except I had to return my rental bike before the shop closed. I accidently went down Bo's Loop, then turned around, confused as to how to get back to the parking lot.
Here's a link to another nice map from someone else's website.