I'm 78 and yes, that's a problem. I think it's more a question of long-term health and injury prevention that it is a question of performance on the bike. My wife and I have been strength training Tuesday and Thursday for decades. I used to ride before gym, but that doesn't work anymore. One of the things I did in response is to start doing 1 set of 30-40 reps to exhaustion. So that's rather anaerobic and for a longer period that short sets. Works and helps make up for the lack of bike time. We're in the gym for 45'-60'. You can try an hour of cycling immediately before the gym, see if you can tolerate that. I used to be able to do intervals and still lift heavy.
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