It's hard to say without knowing what your maximum HR is (I mean flat out maximum, not maximum from a normal ride). But 170-180 for most people would be close to going into the red. The fact that you held it for an hour and didn't blow up suggests it was at least below your threshold, but probably close to your limit. Unless you have a heart condition, there isn't much "danger" involved in trying to hang with the B-group. If it's too hard you will just blow-up at some point and get dropped. Or you might scrape through to the end and feel completely exhausted. Both of those cases would tell you to spend longer in the C-Group.
I would also say in general it is better to do your longer rides (2+ hours) at a moderate pace and push harder on shorter rides (1 hour or less). It's just more productive that way if you are aiming to get faster in the long run.
Last edited by PeteHski; 06-25-22 at 03:13 PM.