A precursor to mountain biking, cyclocross differs from its more popular offspring in a number of ways. A true 'cross bike employs the cantilever brakes and knobby tires of a mountain bike, but on a road-style frame with drop bars, reversed brake levers and 700c wheels. And about a third of the typical 'cross course is designed to be covered on foot at a dead run, carrying the bike over obstacles, up hills and through muddy, open fields.
The best thing about a cyclocross workout is that it's rarely dull. The wide variations in terrain and weather conditions will keep your brain occupied while it's searching out the best line. It will give your heart a workout as it peaks and plummets like a roller coaster. You'll go anaerobic for short stretches while powering over short, muddy rises. You'll run uphill with the bike to get some lactate-threshold work on the flats and then recover on descents.
If you're not running already, begin with short runs at a moderate pace on level ground, preferably in woods or pastures, or in a city park. Gradually introduce interval-style efforts, including jumping rocks or logs and uphill sprints; cyclocross runs are short, sharp efforts, not long, steady jogs. Keep the sprints down to about 10 to 15 seconds at first, then build up to 30-60 seconds. Once you've got your running legs under you, try running with a bike on your shoulder.
You should practice your riding on a course that has all the ingredients of a cyclocross circuit uphills, descents, hurdles and mud. You must train to handle a wide variety of conditions and situations with minimum effort and maximum fluidity.
Sections that you can bull through on a fat-tired mountain bike will require more finesse on a cyclocross bike. Here are two tips for getting you through the rough patches: Keep up your momentum. You must always be searching out the best line down the trail, when there is traction to be had, and then applying lots of pressure to the pedals, in a gear that's neither too high nor too low. You don't want to get bogged down in too big a gear or let your rear wheel spin because you are in too low a gear.
Keep your body and pedaling motion smooth. This also applies to mounting, dismounting, cornering, dodging obstacles and braking. Keep them all smooth and you will waste less energy, saving it for those spurts when needed.