The Fullerton Loop is a fairly popular ride right smack dab in the middle of Orange County suburbia. While it's not going to win any awards as the next big mountain bike destination, this 11-mile loop right in the center of Fullerton is easily accessible to the residents and makes for a great training ride or a great loop after work.
Utilizing a number of different trails and greenways and unmarked sections of singletrack, the loop is surprisingly easily to follow. There are a few spots where you might need to look at a map to figure out which road to take to connect back to the trail, but for the most part you just need to look for the bike tracks and see where they go!
While most of the main trails are wide, multi-use doubletrack, there is a ton of singletrack options off to the side, many of which are well-defined and are even marked. There are a few jumps and drops thrown in at intervals to keep things interesting, as well as I few berms/wallrides, including one that's about 10 feet tall!
If you live in Fullerton or are staying here for a while, you should definitely hop on your bike and go check the trails out.