I'm in almost the worst shape of my life but far less depressed about that than when I was 33, enjoying an incredible year of training when I had finally learned how to train properly and was back to running after 6 years off. I had the feeling like 'I wish I had more time before the wheels fall off'.
Now I know I will never be able to do even a 10k race at the pace I used to run a 20 mile training run. So I feel no sense of worry about not making progress. I know that I can start training tomorrow and enjoy literally years of my health and fitness getting better. It might top off with me running a 10k at 7:00 pace but that would be like aging backwards at this point. You can't get that sensation when you are 25 and in the shape of your life.
Health has become a bit of a concern. I'm less worried about my current lack of fitness (easily fixable) than I am about the cumulative effects of 3 decades of terrible sleep patterns and too high stress. But there's nothing to be done about the past, so just do your best and be grateful if you're in decent health.
My father was never an athlete during any period of my life. He'd been a boxer for a little while when in high school, and probably would have been a fantastic endurance athlete, but he grew up on a farm and never had time to do it. He enjoyed watching sports but didn't really participate. Physically, he was a very hard worker (he enjoyed yard work) and we had the nicest imaginable yards in the houses I grew up in. He literally spent hours of his time every weekend landscaping. I recall him coming back home (he was working out of state) when I was a very fit PAC 10 runner and he'd make me tired working in the yard with him (he was close to 60 at the time). He was in fine shape and only slowing down a little bit until 85. He's 88 now and has been essentially bedridden for the last 2 years. So no matter how fit you are, time will eventually catch up with you. But he enjoyed a very physical life for 85 years and so can all of us if we don't have some unfortunate illness or disabling injury. I 'm hopeful that modern medicine will have better anti-aging treatments in the future; it's looking unlikely for me, but people in their 20s-30s have a lot of hope to be more physical at a very advanced age.