Yep, all us ol' codgers are going throught the same thing. I agree with your philosophy - do it today for tomorrow may never come.
Personally, I've suffered through some tough years when bad boots were chewing up my feet or knee problems were making every step painful. But, I've never let any of that stop me from enjoying my passion. Be careful not to hurt yourself, though, and hunting with a buddy is a great idea.
But don't assume that this is a one-way slide - it's possible to see some improvement from one year to the next, even if we're one year older. 7 years ago, I bought a bird dog. I resolved early on that I would ensure that the dog was well cared for, including plenty of exercise. I'm here to tell you that, if your dog gets plenty of exercise, you get plenty of exercise (as long as you gon't exercise your dog by having him/her chase after your truck).
I've learned from experience that plain ordinary walking is not just a good exercise, it's the BEST exercise for gradual improvement in overall health. Get yourself a pedometer and find out for a fact how much walking you do in a day. The consensus is that you should be walking 10,000 steps per day (the average is around 5,000). Gradually increasing your daily steps while being careful not to overdo it at first will make a remarkable difference. Taking your dog out for a 1 hour walk every day is usually enough to put you over the top. Furthermore, if you like hunting the uplands, your dog will amaze and delight you in the field.
Needless to say, if your lifestyle includes a questionable diet or really bad habits, you might want to get those under control, too. But, my opinion is that what you have been given is a "wakeup call", not an irreversible verdict. Yes, you need to adjust your lifestyle. No, it does not have to mean sacrificing the things in life that you enjoy most. But, you have to decide what's really important to you and act accordingly. Same old, same old, isn't good enough anymore.