What do you struggle with? Aches and pains?
Some aches yes - shins and ankles. Beyond that, my legs are just too weary to give me as much as I know they can when fresh. I think I'm just over-doing it slightly, even on my modest distances and speeds, given I'm quite new to the game, again.
I have a yearning to get out though - I've had to stop myself going out for an evening run before when I've been out for a morning one.