I had to look up what a plantaris muscle is. Never heard of it before.
You need to stop playing tennis for a while, perhaps see a doctor, and slowly start doing some rehabilitation work. I've become a believer in weight training for tennis, especially for people who are aging. Likely you've lost a lot of strength and even a basic strength routine will help build muscles and strengthen tendons.
Once you feel healthy enough to start playing again, see a good coach or perhaps video tape your performance and look for technique errors. Start slow and work your way back into it. I see guys in their 80s still playing tennis, so I think it is easily in the reach of most guys to play through their 50s assuming no degenerative diseases.