Certified Master Scuba Diver here! Welcome to yet another expensive hobby, lol.
In actuality, your poor air consumption is far more likely to be related to your lack of diving experience and (likely) being overweighted due to that inexperience. New drivers tend to require more lead on their weight belt to keep their buoyancy under control and every ounce of lead means higher air consumption.
I ride 7-8 hours a week and take dive vacations all over the world. The more I dive the better my air consumption gets and my fitness on the bike doesn’t seem to affect that. My advice is to get certified and dive all that you can!
Divers Alert Network (DAN), the preeminent authority on diving medicine, has presented some evidence that increased aerobic capacity can increase the likelihood of decompression sickness. So when diving multiple times per day over the course of a week you want to closely monitor your nitrogen loading and allow plenty of time for off-gassing.
I’ve also read some research that suggests it’s not a good idea to do strenuous exercise immediately following a dive. The recommendation, if I recall correctly, was to allow at least 24 hours post dive. Usually not a problem for me when I’m on a dive trip, LOL!