I have been doing 4 x 8 min intervals ( 2 min rest) for several weeks now, mostly with the aim of improving my 12 mile TT. Do these need to be progressed to continue to be effective? I have been doing them based on threshold heart rate because my power meter broke, so I think even if I were getting stronger, this would still be a useful training target. I don’t need to make them longer etc., correct?