See if you might borrow a few bikes from friends, if no time call Performance Bike in Sorrento Valley see if they can help out. Fun central ride: is Penasquitos Canyon plenty of passers by if you get a flat or some other unlikely mishap I always enter from the West end. I think front suspension hard tails are the way to go (less energy lost), full suspension is for downhill riding and overkill for trails IMO. its a fun activity and a cool run/hike area too http://www.mountainbikebill.com/LosPenDelMar.htm
