Do you train strength? When you are in your late 30s you should really dial back on the running and train more strength. I am 45 and wished somebody told me this when I was at your age. Having a job where one sit all day plus only running which involves movement in the sagital plane only is a bad combination. It may cause weak abs, glutes, hip abductors and upper back muscles which can result in reduced mobility and pain in hip and shoulder.
If I were you I would run only twice a week and then do strength training twice a week.
Regarding which strength training exercises to do:
the minimum is:
Romanian deadlift (RDL): trains entire back of the body but hamstrings and glutes in particular.
Plank: trains abs and most importantly the deep abs.
Sideplank: should ensure flexible hips and keep you from getting pain in knee or hip.
Bent over rows or seated row for upper back.
- Push-ups: less important than the others. Your horizontal pull should be stronger than your horizontal push. When your back starts getting strong from RDL and rows you can make sure your push is also getting stronger by doing push-ups.
As you see the most important muscles to strengthen are:
the muscles around your hip (core) and the entire backside of the body (posterior chain).
Another alternative is to follow the starting strength program and add in plank and sideplank. I have followed this for a 1/2 year and am stronger and have lost centimeters around my waist.
The hamstrings tend to be especially weak in runners:
http://www.fleetfeethartford.com/sports-medicine/sports-medicine-corner/hamstrings-strength-flexibility
Also runners tend to have strong hip adductors but weak hip abductors and poor external rotation in hip:
https://runnersconnect.net/hip-strength-and-running-form-the-role-of-hip-drop-in-running-injuries/
The sideplank is supposed to be the solution for this:
https://deansomerset.com/planks-are-the-magic-bullet-for-hip-mobility/
I have personally experienced poor external rotation in hip when doing kickboxing and back squat. Currently I am doing sideplanks and seem to be doing good progress but have only been doing this for a few weeks.