I can see some of what you're talking about in myself. I'm not sure it's all age in my case. Over the past few years, I've tried doing some of the same workouts I did 10-15 years ago and it was like beating my head against the wall. While I think it's as much the lack of base I had 10-15 years ago as age, though I'm sure my body isn't quite as resilient at 35 as it was at 20 or 25, I can see this. That's actually why I generally avoid the track these days. I don't want to get on the track and get sucked into trying to hit splits I would have 10-15 years ago. I'd rather just convert those workouts to time-based workouts and do them on the road or trail.