I would have to question how you arrived at your max HR number, because a person does not generally do 2.5 hours @ 85% effort and feel fine afterward.

I think my last FTP test, including warm-up and cooldown (total moving time ~90 minutes,) only had a 0.79IF, and I was right knackered afterward.

Have you by chance done an LTHR test to determine your threshold, or are your zones set from the 220-age recommendation?