The Vortex (McCosker trail + tributaries) is a set of trails well suited for mountain biking in the north end of WildCat Canyon in Richmond CA. It is easily accessed via Bonita Road in San Pablo, or via the WildCat Canyon/Alverado park trail system. The views are amazing, and are accompanied by a hefty breeze, so bring a jacket. 
This trail set allows for lap- repetition, and lap-reversal so you can time yourself for training purposes. There are switchbacks, s-bends, a steep descent, a climb or two and a few different sets and orders in which you can ride the trails. This gives you a few different sections and difficulty for your lap training if your so desire. This is also a great location for group riding! 

Ride conditions: Canyon clay, eucalyptus debris. Avoid when wet.

As always, be courteous to the locals, and leave a good impression for the rest of the cycling community!