I'm 68 and some years ago (5+/-) and have had a couple of bouts with tendonitis - tennis elbow from erging and achilles tendon from jogging. I can trace these back to overuse, not warming up thoroughly, and starting back at an intense level after a few weeks of layoff (not dropping back the intensity and adding to the intensity in a measured way).
My tennis elbow was caused by trying to get additional exercise while erging, doing wrist curls, either up and down on every stroke. It seemed like a good idea at the time (and it still might be if not overdone), but the pain got so intense after a couple of weeks that I couldn't even hold a milk carton with that hand. It took about 6 weeks before it went away. Rest was a very important part of the therapy, as well as massages to stimulate blood flow in the affected area, keeping the area warm using sweaters or sweatshirts, also to aid circulation, and stretching the tendons several times a day. The key is to increase blood flow. Keep in mind that areas like the Achilles tendon have relatively poor circulation and massages and warmth help here.
It seems that you trained mostly at very high intensities, trying to pack as much of a workout as possible into the limited time you have, possibly neglecting a thorough warm up. I used to be able to get away with this type of thing until about my mid 50s, but now I wait for the heart rate to comfortably increase before launching into high intensity work. Like MontanaAndy, I've learned to listen to my body and when I feel some pain coming up, I typically ease back until it goes away, and continue at lower intensity where I'm pain free, increasing it if pain free, and stop if it doesn't go away. It pays to be a chicken at times so that you can come back and fight another day.
