You are either trying to move your power UP or OUT, as in doing 20min at progressively higher power or doing the same initial power but at longer times, 30-40-60mins.
TR tries to both at the same time. Take SSBMV for instance. First workout is 3x12 @85% and last workout (in SSBMV2) is 3x30 @90%.
A related thread to read through with some great comments: