I'm going to assert, AGAIN, that mild strides are one thing, inserted into a run. However, for aging runners who have decades of miles under their belts, any sort of sustained speedwork at anything approaching Daniels' R and T paces is a recipe for disaster.
Ask me how I know.
Muscle mass has decreased, small previous injuries have accumulated, muscles (especially hammies and calves) become 'brittle', recovery becomes much less effective, there is a REASON people slow as they age, and you cannot beat it, only work within it.
Instructing the older runner to add track work, repetitions, even too much racing, to their regimen is greatly increasing the chance they'll be sidelined. As far as posters denigrating the old-man shuffle, YOU TOO will be doing the old-man shuffle if you stick with running into your 50s and beyond. Those of us still out there are STILL OUT THERE, healthy, running (or shuffling as you call it), enjoying running for what it was and still is.
You old cranks who still want to do track work, good on ya. Eight out of ten of you will be injured and on your bicycle or couch soon enough, and the shufflers among us will still be out there running, albeit slowly.