How do I improve my cycling at my age (77 years) I am 77 years old and newly come to riding a road bike. In the better weather I do between 75 and 100 miles per week, consisting mainly of fairly easy roads with four or five reasonable hills, depending on length of rides. I do routes of 17 miles, 23 miles and a Sunday ride of 40 miles.

I average speeds of about 14 miles per hour and can get up all the hills but fairly slowly, I have trouble standing up to rest or get a boost in climbing. I do stretching and power exercises and three lots of HIT on an exercise bike each week. I cope with my mileage OK but would like to improve my hill climbing.
You are already doing quite a bit of training.

What might help is training on hills specifically, or when on the trainer, raising your front wheel to simulate being on an incline.