anthmart, I think that you should get some advice from someone experienced in your problem. I don’t think many on here are doing that quantity of intense exercise. I would talk to a doctor, coach, or sports scientist if I were you.
As an aside 15 years ago some friends did a rowing marathon and were advised to eat a high fat diet for several weeks in advance of the event to train their bodies to burn fat. (It is much harder to eat and drink when rowing than cycling.) They weren’t trying to win, just do a reasonable time.