I prefer shorter fasts, otherwise it is just a huge pain in the ass to get all the calories in.
When I do longer fasts it'll be a once in a while thing, I typically go 14-16 hours and every now and again I'll do a 19-20. MOD will say to make the IF, well intermittent! If the IF is affecting your training, definitely cut down the IF or vary up your schedule.
I found I got the best results when I wa able to do this:
IF 5 days a week at 15 hours, no IF on the weekends.
"And for crying out loud. Don't go into the pain cave. I can't stress this enough. Your Totem Animal won't be in there to help you. You'll be on your own. The Pain Cave is for cowards.
Pain is your companion, don't go hide from it."