I'm 57, started seeing accelerated age-related decline a year or two ago, mostly in recovery. Reached a point where I felt crappy on most runs so I've cut back from 6 -7 days a week to 4 -5. Less definitely feels better.
I used to do one or two track sessions a week (e.g., 6 - 8 x 600m w/200m jog or 3 x 1200m), but lately I've started doing the following once a week instead: 2 miles brisk warmup, hitting about 6:00 pace by the time I get to the track; 8 x100m strides, good effort, concentrating on form; then I put on spikes and run 1 x 400m as fast as possible in workout conditions; 2 mile cool down.
This doesn't require a lot of time but done well, it's taxing and I feel really good afterward. I really think it's helped me maintain decent form as I stride toward decrepitude. Breakdown of the stride is what always gets us oldsters in the end.
Don't know if I could still break 5:00 in the mile as I did a couple of years ago, but I think with a short period of increased training I'd be close.