I love mountain biking. The first time I did it (about 20 years ago), I felt 12 years old again (I normally feel about 14). Going over little jumps and leaning into turns. Great stuff.
Penasquitos is a very easy trail, and is easily suitable for your first ride. Check for ticks when you’re done, though. They climb to the tips of grass and hitch a ride when you brush by. If you can stay clear of the grass, you won’t get any. (Contrary to popular belief, they don’t drop from trees.)