Every weekend for several years, I used to take my young kids to this quiet unassuming local park with slides and swings, next to a popular dog park along a cul-de-sac, nestled in front of this hilly terrain with some short hiking trails. Occasionally we'd hike up the short steep trails, but I didn't think much of it. One day, to my surprise, there was a local mountain bike race going on here called the Glendora Knobby Time XC Grind, part of the Triple Crown MTB Series, complete with booths and sponsors. Few days later I rode the same race route, and man was it a grind! That super steep climb after the DH section is a lung-burner. I've used it as a loop for XC training, but I can't recommend it as the trail is overgrown for most the year and there's too much poison oak for my tastes.
Location: South Hills Park, Glendora, CA
Ride: Santa Cruz Tallboy C