I'm 54. Strength-wise, I'm probably the strongest I've ever been - I've packed on a lot of muscle this year. Cardio-wise, I can hold my own but have been much fitter - there's a fair amount of room for improvement. Not to brag, but most people who know me would say that I'm extremely fit - fitter than most people much younger than me. Want to know my secret?
Find something that motivates you to stay fit. For me that motivator is climbing/backpacking/hiking. A long time ago, I learned that I really don't enjoy going to the gym, running, etc. just for the sake of staying fit, looking good, or whatever. Lifting sucks. Running sucks. Eating healthy sucks. However, what *does* motivate me to do all those things extremely hard is knowing that it enables me able to do the things that I love.
So, for example - next August I have an epic climbing trip planned. I intend to climb the Teton Traverse - i.e. I'll climb all but 1 of the major peaks in the Grand Teton range, and I'm going to do it in 3 days or fewer. So this past May, I started hitting the gym harder than I have in a long time. I'm constantly visualizing the climb while I work out - and it gets me really amped, making me want to lift *all* the weights.