Hi,
I am 47 this year. And from my experience, picking up a sports may be more fun and less monotonous. (just my opinion).
I used to swim a lot (doing freestyle laps) on my condo pool. It was really monotonous and I was really bored. I could do countless laps but after a while, I stopped.
I took up golf for many years but where I live, it is scorching hot and humid and I gave up, though if you walk a 18-hole (pulling your golf bag along), it is pretty good exercise (estimate distance cover around 7-8km)
Finally, I took up tennis. With help from an avid tennis lover friend, I managed to pick up the game (topspin swing, leg work etc). And I brought in a coach to provide lessons for my wife and 3 daughters to learn the game as well.
Today, I mean every Sunday 3-6pm if the weather permits, the five of us will spend our time at the tennis court (a.k.a family time). I make an effort to run and chase for the ball. Sometimes we play on Saturday too, depending on court availability.
And I have two degenerated disc on my lower lumbar. I have to wear a girdle to play tennis though. I injured it during my 3 years serving in the military service when I was 21 years old. And yes, if I overly exert, it is painful.
Sports like basketball, tennis, badminton are great avenues to consider. Squash is not bad but can be tough on the knees.
Hope this helps.