Figure out what your limiters are. Do you really need a sprint right now? Would you lose a Cat 5 race because you didn't have enough snap in the last 50 meters? Or would it be because you didn't have the fitness to bring your sprint into the final 50 meters, or even worse, didn't have the fitness to be in position in the final 50 meters?
Read up on sweet spot training, and do that. If you really have race limiters in intensity, then you should start working 5' VO2Max intervals into your training, and when you get close to race time, shorter anaerobic work capacity intervals (like a 1125211 pyramid or 6x1 intervals, with 5' recovery between all).
Doing a double century doesn't have to be slow... I averaged nearly 20mph for my last one.
I think that actually proves the point. While that's "fast" for a 200 mile ride (congrats, seriously
), it's not "fast" in race terms. My threshold intervals are in the vicinity of 23-25 mph. While that's not enough to get away in a P/1/2 race, it's good, hard training. Endurance riding is important, but it's slow, so it's only one ingredient in the recipe.