Link to trail map:
This map is slightly out of date, but that's a good thing. New park rules: all firebreaks and multi-use trails (except the fitness trail) are now open to mountain bikes. There is a park employee actively advocating for mountain bikes, who also leads weekly rides.
Mostly fireroad and double track. Not much in the way of true single track, and not too much technical difficulty. Some loose rock on the trails, some washouts, and a few erosion control logs. BUT the hills are a good workout, there's a lot of wildlife, the park is environmentally unique, and there is minimal traffic on the trails. You can ride for hours without seeing a horse or another rider.
Trail heads are at the east and west ends of the parking areas. I like to start at the west end, loop the outer trails counter clockwise (about a 5-mile ride) then head into the interior trails. You can cross connect the marked trails and the firebreaks in many different configurations.