exercise and fasting

What are your recommendations for exercise and fasting? Or maybe I am still just getting used to this fasting thing. I typically exercise in the morning for a minimum of 30 min (typically high level exercise) but can be up to an hour and sometimes 3 hours on the weekends. 1- Specifically wondering about fasting in the morning when I do the long exercise times (also in the morning on the weekend). Should I switch my eating window from 12-8 to 8-4? Do this just on Saturdays? Do this all the time? Any other ideas? 2- Trying to exercise this week and do the morning fast I feel pretty grumpy and difficulty concentrating. While I ate healthy in general prior to this is it the exercise + fasting? Or is it just dumping the sugar/flour?
Maybe I just need to experiment… Thanks!