If you're in your early 50s, you may be only halfway through your adult life.
Children born in the last few years in first-world countries are very likely to live to 100. If you're in your 50s now, the odds of making it to 100 are reduced but your chances of being around and reasonably fit in your 80s are certainly strengthening.
So with your chances of being reasonably healthy for another 30-plus years, isn't it worth giving some thought to what you're going to do with all this extra time?
For a start, it's likely that you'll have to work longer. Unless you plan to be very rich, it's hard to finance 30 or more years in a comfortable retirement. It might be a good idea to start thinking seriously about what work you could do in your 60s and 70s and more importantly, what you'd like to do.
It's not really surprising that so many people in their 50s and 60s are becoming entrepreneurs in areas that interested them. But a word of caution - it's still much easier to fail in a new business than it is to succeed. But if you've had 30-40 years of commercial experience, you should be able to avoid a lot of the mistakes that lead to failure.
Outside of your future job, what would you like to do with the next stage of your life? Before getting into the nitty-gritty, it's worth considering that one of the most important things that will keep you happy, healthy and interesting is to have a purpose in your life. Purpose has more impact on your wellbeing than regular exercise and a good diet. Find something you're passionate about and go for it. Make a difference - it will keep you young.