BE HEALTHY!!! Having a strong and fit body not only improved your game tremendously- but also will allow you to keep playing through your 80's!!! When I started playing tennis in Austin at the #courtyardtennisclub in 2009 - I weighed a whopping 195 lbs (on a 5' 6" frame). I quickly noticed that as I lost weight - I felt better on the court, I was faster and I didn't get injured as much!
I dropped 50 lbs by decreasing my calories (the easiest way for me was having a very low carb diet < 20 grams per day eating WHOLE foods), increasing my movement (I played tennis 5+ times per week), staying hydrated with WATER and herbal teas (makes you feel fuller) and meditating daily for 30+ minutes.
When my body is fit - and my mind is calm - I play better - I love harder and have loads more FUN!!!