Most of my riding is either DH or trail along with some DH/DS racing but after watching a few cyclocross races i think i want to give it a shot. Although I work on my cardio to increase my DH times it's nothing like a strong roadie or XC racer. So if i'm going to race at cat 5 (which is like a 30min race) I was wondering how to gauge how far i need to push myself to train for cyclocross. Some way to know if my cardio is on the level of the rest of the beginer racers. (ex.being able to sustain 80-90% MHR for X amount of time?????) I don't have a CX bike yet or something I can ride to train with other CXer's just my enduro SX with a single ring but regardless I'm only concerned about my cardio for right now. Any ideas?