Lake Poway is a singletrack loop around the lake and behind the Damn. around the lake is a nice XC ride and behind the damn is a nice DH with switchbacks. once your at the bottom you have to ride up the other side. ugh.
Review: Go early in the morning during the week to avoid heavy traffic. The trail up is brutal. the down hill is fun and extremely fast. If you want you can go all the way to Potato Chip rock. Just be warned that you will need to walk/carry your bike at some spots.