I'm 75 and afraid I don't have much advice for you. You're still fighting the aftermath of Covid, which most of us haven't had to do. Guess I would recommend starting with little steps. Just walking. Start with one walk a day. Then perhaps two. When I was approaching 65, I decided to get another dog to help keep me in shape. I like bird dogs and acquired another Brittany. He did what I asked and more. At least one fairly robust walk every day and what I call "wing work" in the yard. Kept me active. Unfortunately, he contracted cancer and we had to put him down last summer. Still walking though. I have a set of weights getting rusty in my basement. I know I should be working out with them, but find it very hard to bring myself to do so.
I am planning another western hunt this fall. The goal helps with motivation. I'll continue to walk farther, faster and on more varied terrain to get in shape. At the same time, I recognize i won't be able to go as far or as fast when I get out there. I've packed elk three miles. That won't happen again. Though i can work that far away from camp fairly easily, I know that I and and a hunting partner of similar age, would have a helluva time getting an elk out that distance before the meat spoiled.
It's very difficult, but start with little steps. When you begin to see/feel a glimmer of improvement, that, in itself, serves as motivation to keep on going. Good luck.