One thing I can definitely say I have noticed as I have started to ramp up in this training program: things don't respond the same way they used to! I'm 45 now, which ain't old (or at least you'll never get me to admit it), but I'm definitely *not* 25.
The biggest difference? How long it takes for the soreness to show up.
Used to be pretty simple. I'd go out and train long, or hard, or in a different way. The next day I'd be sore, and I'd say "yep, this is from yesterday."
Now the time lag is more like 2 days, which has taken some getting used to. Say I do a big mountain run on Saturday. Sunday I still feel fine, so I think "great, my condition must be pretty good." Monday morning it's a struggle to get down the stairs, and I'm all confused because I didn't do anything *yesterday* that should have made me sore.
But I'm getting used to it, and I'm training myself to look back more than a day in my training log when something hurts. This helps everything make a lot more sense.
There's also a positive spin to this, which I'm gonna keep repeating in hopes I will eventually believe it. :) I can now OUTRUN PAIN!! Look at it this way... it takes 48 hours, or at least a good full 36 hours, for the soreness from a hard run to show up. OK ... the 100-miler I'm doing has a 36-hour cutoff. Do the math. This means that I will finish before I get sore.
It's magic. I'm now old enough that I can outrun pain. Yeah!