You should really be fine on all the trails at Alafia, but you'll want to take it easy your first time through on Gatorback and Moonscape. Gatorback has the biggest downhills, but they are generally smooth and have good run-outs. Gatorback also has some of the biggest climbs - they are all doable, just takes some practice. Moonscape has some more technical sections with rocks that can be tricky until you know where to put your wheels.
Start out on North Creek as it is a great warm-up.