Been on a new thing - very slow running. Goes by many names: Low heart rate running, MAF, aerobic threshold (AeT) training. But the general idea is that you are running (or biking or whatever) at a pace that keeps your blood lactate below 2 mMol/liter. Since I don't have a lactate monitor (!!), I use the MAF guideline - 180 minus your age. For me that means keeping my average pulse below 124. Which is basically crawling for me.
The idea is to train your aerobic systems rather than lactate threshold (LT) training that trains LT and VO2 Max, but trains the aerobic system mostly by accident.
LT - increases stroke volume, lactate clearing, that sort of thing. So you can stave off depletion, but you are depleting your glycogen stores and when they are gone, you bonk. It doesn't matter how much lactate training you do, you will bonk if you go far enough.
AeT - basically is increasing mitochondria and, therefore, your ability to tap fat reserves, so you are not depleting your glycogen. So you may experience extreme muscle fatigue, but you don't bonk because your fat reserves always have more fuel than your muscles can sustain.
Anyway, I've recently become aware that the endurance athletes that I admire have been doing this for year and all this time I've basically been running too hard. This goes back to Mark Allen's victory in what is considered the greatest Ironman race ever run where he beat Dave Scott. This came after Allen spent six months doing slow running under the guidance of Phil Maffetone, originator of the MAF system.
Anyway, it's humbling, frustrating, difficult... but I'm really enjoying it.