Of course it depends on what you are trying to improve. If you have rolling terrain you can ride hard up the hill and take easy on the down and between. It sort of works out to work time = recovery with some variation.
If you want more of a muscular-endurance workout, I've done a lot of "crisscross threshold" rides this year. Warmup to lower zone 4, increase to upper 5a in 2 minutes, decrease to lower zone 4 in 2 minutes, repeat.
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