Interesting to me that you mention that. I've been doing a polarization of easy efforts (75-80% maxHR?) and hard efforts (90% maxHR) on rollers and I am finding my legs far more achy towards the end of the hour-long easy sessions. Sure, the hard sessions with the 4x8minute intervals are challenging, but my legs don't really ache (though they feel more tired the second day) even though those hard sessions last as long as the easy once warm up, cool down and recoveries are added in. But the easy efforts, which are really easy from a cardio point of view, become challenging starting around the 55 or 60 minute mark because of my legs. I kind of wonder if it is a saddle issue as the ache seems to start around the sit bones and then adds in the hamstrings and then the calves and then seems to be everywhere.