Reading all these muscle injuries I think of two things. Hydration and stretching/warm up.
I definitely drink more beer then water before and during games. Us old men muscles need to be extra hydrated and warmed up before any physical activities. Unless you are just a catcher and need a runner after every hit some pre game warm up is needed.
Hydration is a HUUGGGEEEE factor as you get older. I'm 40 now, and when I was in my 20s or even 30s I never gave hydration a thought. I'd play all day and drink almost nothing.
Now I make a conscious effort to drink lots of water and powerade (mostly water). I'll even mix in some powdered hydration booster at times. My body lets me know quickly now when I'm not hydrating enough. I'll get stiff almost instantly and simply feel dried out.
Only serious softball injury I've had was a pulled hamstring 4 years ago. It hurt like hell, and I knew I had an instant problem. Light bruising. That injury kept me off the field for 6 weeks, and even when I returned I wasn't 100%. That hamstring hasn't bothered me since. I at least partly attribute the hamstring injury to wearing crappy shoes. I bought a cheap pair of turfs from SIS one year on a black Friday deal. The shoes were horribly uncomfortable to the point I feel they affected how I walked or ran on the field. Even after the hamstring injury I wore the shoes a couple more times, and had continued general leg pain. I finally wised up and threw those shoes away. Now I play softball in tennis shoes. I'm willing to sacrifice traction for comfort at this point. I've had no leg issues since, and average New Balance shoes do just fine on the field.
Getting over the hamstring injury kind of sucked. I made sure to stay active, and walked on it even though it hurt. I feel like using the muscle (lightly) had far more benefit than simply sitting around letting it atrophy. I bought a neoprene hamstring compression sleeve that I wore at night. I feel like that helped the healing process along.
I don't stretch excessively before games, but I will run a little. Usually some sprints followed by a jog around the field/complex. I feel that adequately gets my legs going without completely tiring me out. Simply jogging has made a difference. I can run the bases or OF and feel good, not worrying about pulling something.
Final thoughts: At 40, my body tells me when something's not working right. If I'm sore or achy, I'll take it down a notch rather than continuing to push. I feel like I can still play softball at a very high level, but I have to think about it more. You really do have to start putting thought into how to reduce the risk of injury as you get older.
I'm really surprised how many guys I see playing who are my age or older who don't do anything to warm up before a game. These guys are always getting hurt, and it's almost always pulled hamstrings, quads, or calves. A little preventive maintenance goes a long way. You have to run around a little before the game to get your leg muscles working.