I don't see the rock and hard place. You have a fulfilling relationship that is based on compatibility and emotion. The problem is that you can't project your fear of aging onto her. She may be very comfortable in her skin and that is why she does not find it necessary to exercise. If you love her, then you should respect that choice. If you look for the greener grass it may not be the lush lawn that you think it is. You have a person who you love. What difference does it make if her body is slightly more fit than yours. It is superficial to expect someone to put emphasis on their body when that is not how they feel and is not a priority. Her priorities are what they are. You may try to make the gym a couple's hobby but if she doesn't want to go then she doesn't.
Focus on educating yourself about this time period and how you are viewing this period of your life. That should be your focus. Concentrate on you and your fears
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