The "speed work" I've done trying to improve my speed was always short intervals with a long recovery, say 30" - 1' at full gas, then 5' recovery, repeat until it's obviously not working anymore, like 6 anyway, something like that. And google "cycling speed work intervals". Lots of various ways to approach this. That said, these are not sprints. Sprints are very short, like 15". Whole different deal. I've enjoyed training both things and each of them contributes to performance in the other.
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