^^^Great advice @stanley!
I think everything has already been covered above.
Diet is the obvious one, growing up, my Mom used to tell us that you are what you eat. This is so true, food is fuel and if you want your body at optimum levels then you need to fuel it properly.
As far as exercise, find something you really enjoy, gradually increase the distance and intensity, go as hard or easy depending on how you feel on any given day but also think about recovery and how you can improve it.
After the first 100 days Challenge, probably about 4 years ago, I made a deal with myself that I would try to exercise for an hour every day with the hope of maintaining a decent level of strength and fitness in my next half century.
I run, cycle, do weights and resistance training, mix it up depending on how I’m feeling but over time I’ve learned that rest and recovery is really important. To aid recovery, walking is essential, I stretch every day and use a foam roller to iron out any niggles.
Exercise is my therapy and I love the buzz of endorphins. I have just formed a habit and it is now something I do without thinking.