One rational approach is what I have been using in my triathlon (and has been very successful for me, and even more so now, I guess) is Heart
Rate based training
. This means that I nearly always work in my aerobic zones
. For me, this means I keep my heart rate less than 140 bpm (usual target is about 134 for a moderate effort, but for an easy effort my HR can be around 120 bpm - note that my maximum HR is about 180 bpm and I have achieved this in all out efforts).