I've been riding in Forest Park lately. There are only a few trails where bikes are permitted, which is frustrating considering the park is 8 miles long and 5,100 acres of mostly trackless forest. But there is a 11 mile winding trail/access road, another trail that is a solid climb, and several steep single track "fire lanes" branching from those trails. And probably more, I've only recently been exploring this park by bike.
Lots of people ride this park on cross bikes. I saw a guy come up out of one fire lane on a cross bike, going fast, not red faced or wasted. Then I went down that fire lane on my mountain bike, and had to walk a couple spots because they were too steep, tight, and rooty for my rusty mountain bike skills. Oh well, sigh.
Today I rode 5 miles to the trailhead, about 15 miles on the trails, then 5 miles home. I've been trying out this thing where you ride a big gear at low cadence, standing when necessary, to build leg strength. Fun stuff. The bummer is that after airing down to 35 psi for the trails, the bike is incredibly slow on the paved road home.
That is, by the way, a C&V mountain bike.