Clenbuterol is good. Studies have verified that clen increases muscle mass, decreases body fat and exerts metabolic effects that augment the endurance training effect. (Brooks et. al., 1996) Now, everyone reacts differently, and some people don't like it, most likely because at moderate to high doses there is a "speedy" effect.
In regards to injecting ATP... I'd like to know more about this. I would be interested to hear the results if anyone has tried it. My guess is that it would be similar to high doses of creatine??, as the ATP-CP cycle go hand in hand.
As for training, in the off season, I believe it best to decrease overall mileage and focus on anerobic threshold-type workouts, going at about 80 - 85% max effort for shorter distances with some all-out max speed work. Of course you'll want to throw in a distance day every once-in-a-while to keep the aerobic system tuned-up, but any gains you can make in raising VO2 max will definately pay off. Combine that with a great weight training program, especially for the legs, and you have yourself a pretty cool winter training schedule.
As Spring approaches, that will be the time to up the mileage again, and with your VO2 max up, even a point or two, and you'll notice you can go faster on those long hauls.