All good advice so far...first ask yourself...which do I want to do? Gain muscle? Better Metcon?
If you want muscle: Dump the metcons, lift heavy, recover more, eat more
As mentioned before you can easier get endurance back after the weight gain. Eat as much healthy fats, protein and paleo carbs as you can. Forget IF for now until you really learn how to eat alot in a smaller period of time. Then add in just 1-2 days of IF. IF in the wrong hands or wrong eating will just lead to weight/muscle loss.
Make it a lifestyle program, eat healthy, recover and the body will respond in kind.