Have you heard of Body Balance?
This may be preaching to the converted, others may have their own work-outs that are good for them but this may help the rest of us.
If you haven't heard of Bodybalance it is Tai chi, Pilates and Yoga all in one session and it's great. Improves flexibility and helps prevent injuries and you can see gradual improvement over a period of time. It works well for both men and women and perhaps easier to work your way in to than both Pilates and Yoga.
It's fantastic to see so many people increasing the amount of tennis they play. However many are rightly concerned about injuries and avoiding too much tennis to avoid such problems. Personally until I took up Body Balance 5 years ago I could only play golf once a week (back issues) and tennis (shoulder/neck) likewise - now I have greater mobility and just want more. Body balance and proactive sessions with former England Cricket and Tottenham Hotspurs Physio Kirk Russell have been a tremendous help to me.
If your interested in either of these I've given some links below:
Bodybalance
The course designers - it changes every 3 months
https://www.lesmills.com/uk/workouts/group-fitness/bodybalance/
Video of a Bodybalance class
https://www.youtube.com/watch?v=3lXaYMDxGcU
The health industry recommend
https://www.nuffieldhealth.com/gyms/classes/bodybalance
Courses in Bath
http://www.fitnessfirst.co.uk/classes/bodybalance/
Where I go - classes are included in your membership fee at Combe Grove
http://combegrove.com/club/gym-exercise-fitness-classes-bath/body-balance-class/
Physio - Kirk Russell website
http://www.kirkrussellphysio.com
I hope this and your own work-outs will lead you to being able to play more tennis (if you want to of course). I don't agree with those that feel you can't play tennis once you've reached a certain age or need to limit the amount you play because of your age.
Andy