I would say that unless you are fat, gym work and sprint specific work on the bike is enough to make you fit enough. Though where this really comes into play is endurance in the rounds of a sprint tournament, but I would say even this is taken care of by typical sprint training, for example a session of rolling 500's on 12-15 minutes rest will be pretty good preparation for this (as a side benefit of the main training focus). Throw in some lactate tolerance stuff closer to events and you'll be pretty damn fit. I don't think anyone needs to do three+ hour road rides these days, unless they are trying to lean down. Long stuff will definitely blunt your speed.