Muscle weakness in the hip, hamstrings and glutes can all cause knee pain. Prolonged sitting can weaken all three. Efficient running and doing squats starts with balanced hips. Downhill is harder on the knees than the level.
Don’t know what you mean by not being a young guy, but running form tends to weaken as we get older. With age the knees tend to start going south.
Problem could be related to the hips, or the need to cut back on the running and slowly build up to previous mileage. Or the knees themselves.
I’d start with the hips and cutting back on the running distance, along with having an Ortho take a look at the knees.
Running into middle age and beyond, isn’t for all.
