That is a lot of exercise in a week for most people, unless he's in training for a triathlon or iron man challenge.
Is it more now than it use to be and as you no longer go it's more of a problem for you ? Exercise is addictive, all those feel good hormones rushing about the body makes it very hard to miss a session doesn't it.
I think you need to have a chat with him about how left out you feel now that your abilities are different, it's not easy is it when you don't want to sound life a spoil sport .
Are there any activities you can still manage that you can do together ? It's not an easy task redefining yourself when life throws a spanner in the works, all the plans and expectations of how things will be are thrown out the window.
So no, in answer to your question I don't think you are being unreasonable in feeling disgruntled. If your medical condition has come along fairly quickly and has stopped you in your tracks you haven't had time to prepare for the changes in your life.
Talk to your husband, tell him how sad you are feeling about not being able to share this huge part of his life, it was a shared activity once that you'll be missing as well. Keep talking, keep thinking of how you can still do some things but maybe differently than before, make new hobbies so that you don't get swamped by loneliness .
It would be unreasonable of him not to understand what an impact your medical condition is having on you and adjust his gym times / frequency / length of sessions etc so that he has more time for you.
I hope you feel better soon and can adjust to the changes ahead, you both need to adapt a little to enjoy your retirement