Tennis is a lifetime game but I've started to play seriously at 40.
Please find my top ten tips to optimize performance on the court:

• The most important rule is to rest to prevent injury
• Use elastic and do light weight exercises only off the court
• Ramp up the number hours of tennis per week as long as it stay in the comfort zone
• Focus on the footwork, game strategy and play competitive matches to progress faster
• Review your racket (100in/645cm)/string (23kg/50lbs) setup: Beginners to play with power racket and multifilament strings. Advanced to play with spin/control and poly/gut hybrid. Get two rackets with similar specs (ask matching to your stringer pro) if you play matches. Gut breaks easily under wet condition. Having a spare multifilament is always a good idea
• Eat and drink clean as much as possible
• Focus on one specific thing per training session until it becomes natural
• Active with the legs, relax and whip effect with the arms
• Breath during strike
• Don't follow social media accounts giving technical advices except trusted ones like Laurent Rochette and Pauline Payet

As a final word, you can't know your level until you play official matches and don't forget to trust the process