As someone about to hit late 40's I can give some applicable advice here. I manage two over 30 teams, a coed and a mens team. I play on both and pick up games when invited. I've had a long list of injuries but always come back.
You do need to change how you play, the competitive part has to drop down a bit, basically let the younger peeps have those 50-50 balls, don't go for headers that have any kind of pace, avoid contact with players you know are willing to injure you.
But most importantly, go to physical therapy and collect the appropriate workout. Hit that workout 3-5 times per week and don't stop. If you can't get your workouts in, don't play that week.
I hear you... Man I hear you... I'm more an attacker but what I've found is that I'd rather play with like minded individuals that we all enjoy eachother. We don't win most of our games anymore but we do still score goals and win games. They know that putting direct balls at my head means I'm going to let it go by. But one I can sprint on? Yeah baby, it's a footrace. If you play center back, consider playing as a target forward. A lot of the same skills, but instead of running at players, you're more back to goal, hold up play... Bringing your teammates into play. Finding that clutch through ball for the winger.
If you want to continue this conversation on a regular basis, I'm super up for it. We've got 50+ERS on our team and at this very moment I've been out for two months due to a back injury. You are not alone and I can help navigate.