I agree with what has already been said. A race can be a positive motivator or an invitation to injury depending on your disposition and training methods. The recommendation to listen to your body is a great one.
Like you I'm no spring chicken but I've found when I take good care of it my body will let me train hard and put on a lot of miles. I also swim and bike. From your description I wonder if your training hasn't become stale and no longer leads to improvements in pace and distance. Here are a few training tips that have worked well for me.
I run almost exclusively on dirt trails now which is MUCH easier on my body and, for me, a more fun and engaging environment. I look forward to getting out for my trail runs. I mix up my runs rather then running the same distance and pace each time I go out. Typically I run 3 times a week, one long run, one run at race pace, and one run focused on either speed or strength (hills). My rule of thumb is work on endurance first, then speed only after you've built a good distance base. Also, eating healthy has a huge and positive impact on my ability to train hard.
Running a new course or location is always fun for me. I'll occasionally do a long run or pace run without a watch to run just for the fun of it. Having a commitment to meet someone to run is a great way to get yourself out the door if lack of motivation is an issue. When I had dogs I really enjoyed taking them out into the woods for a run. What's fun for you and do more of it.
So, my suggestion is mix it up a bit with the goal of having more fun on your runs. Finally, I'm currently experimenting with something called Chi Running. Chi Running is a running style that focuses on a more healthy (injury free) running form. The by product of which is an easier and faster pace. So far so good. You might want to check it out.
One last thought. You can always agree to join your friends and train for the ½ marathon and make the decision later to run it or not. Good luck and keep asking good questions.