I think you see this with the longer tri's. In the aerobic zone your body burns enrgy from fat. When you go anaerobic your energy comes from glycogen or basically calories that have recently been processed. You can only store and consume so many calories before you either run out (bonk is the term) or consume too much and get sick. Stored energy in the anaerobic zone runs out after about an hour and a half. Everyone has plenty of fat to run multiple marathons. The trick is to alter your zones so you can go faster in a lower zone and make your glycogen last as long as possible. At higher heart rate zones your stomach does not process calories and tends to make you sick. I have not noticed any lack of initial strength but I am not all that competitive.
Some guys try to lose muscle weight in the upper body to make themselves lighter and be able to run and bike faster.