I do not mean to be antagonistic. But, David, I think you just helped make my point for me -- and you only just turned 40 years and have the best of the over-40 years ahead of you!
You have been injured this year. And, it was when you started increasing your running.
The very fact that you are cognizant of the fact that you cannot run every day, and have to cross-train, suggests you probably are a bit limited by the fragility of age, at least somewhat.
When a person is in their 20s and 30s they do not have to modify their training because they are worried about injury.
I do not know how long you were injured for. But I am going to venture a guess that recovery took longer than it would have in your 30s and so you had to work a bit to get back to pre-injury speed and distance.
That modification of your training, and time spent recovering, is interfering with your improvement. You may still be improving somewhat. But, age is interfering. And the older you get, the more that will be the case. My experience tells me that this interference begins about age 40 years.
-- I apologize if I am seeming argumentative. I am simply trying to add some perspective.
Of course, everyone is different and, despite my whining about being so old I am perpetually limping, I run pretty well, I think. I may not be as fast as I once was. But I push myself when I go out for a run and I love doing it, and I think we all can do that -- still.

Fair enough

God, you old farts get cranky about stuff. Jeez.

I started running when I was 40, and I was a heavy drinker and smoker.

My Mom didn't start running until she was 62. Just this passed weekend at 63 she finished her fastest 5K at 33:30 I think it was. She placed 3rd out of 24 female runners in her age bracket and that's after ONE YEAR of training!

I can only see her getting faster as she runs more.

When I first started running with her, she was much slower than I was. I am 37 and have been running for 2 years now. I am still much faster than she when I want to be, but where's the fun in that. I stayed with her for her first 5K and our first half. She did most of her first 10K by herself and won her age division in that after running for about 9 months. It was a super small local race though.

Needless to say, she has the perfect running gait, which I envy, and I see her getting faster.

I would say, the only thing she struggles with is the mental side of running. But, I think that will come as well.