Back in the 80s I got into Tennis in a big way. I had classes twice or three times a week, 2 - 3 hours each time. Played every other day and was very fit. Starting the 90s, I began working at a full time job, traveling, getting married, and five kids later I'm now overweight and panting when climbing up a floor. Over the past 25 years I think I played just a few hours.
Finally I just moved to a place (an island in the Venezuelan Caribbean) where I have the time and availability to start playing again. My two older kids play with me and they are getting the fever up. I hope they don't do as I did.
Right now I'm plagued with slight muscular aches and discomfort on many places in my body, and they come and go as I play and rest the next day. They are reducing in intensity as days go by, thankfully, but still is annoying not being able to play the way I used to back in the old days. What I felt almost immediately is the flexibility I gained just the first couple of days I played. Definitively that means the exercise is doing its job. I don't expect to recover the level I got when I was much younger, but to be as fit as I can to be able to withstand a full three sets match and walk back to the car not using a walking stick.
My contribution to younger people is to NOT do as I did. Please keep up with the sport. If your personal or familiar issues rob you time, make the time to play at least a couple of days a week. If you can't find a partner, go and practice with a wall. But NEVER quit. You'll regret it big.
To people my age and in my conditions, I can tell you is never late to come back. You'll feel the improvement in your overall lifestyle almost immediately. You won't get any drop shots the first few days, believe me, but in time your agility will improve greatly and you'll be having tons of fun in the process.
Over ALL sports, I can't say enough how much I love Tennis.