Congrats on your good 10K race, Paul--that's a good pace.
Simon, I've been feeling lethargic as well, all last week and the beginning of this week too. I think it's both physical from the race effort and excitement and mental as a bit of a let down. The younger runner who passed you (his first half)--it's hard to chase someone, to catch them, and watch them go by. It could be it was his strategy to go into another gear at the end, if he could. In another race with him (or them), you could have just as easily come out on top. The younger man may have been fatigued at the end, and really struggling, just perhaps not mentioning it.
That's how it is, I think, with people who are running just ahead or behind you in a race. It does become a mind game. I had a similar frustration. In my race, a young woman (25), was running alongside my friends and me in the last two miles. She was struggling with the effort. Her head was lolling, and her form was ragged. With about half mile to go, I told my friends, let's kick it in, and the younger woman, who I had lost sight of, followed. She caught my shoulder at the finish line but I thought I still stepped across ahead of her (the race photos show I did). But the chip time showed her two seconds ahead of me. It turns out the chip ending of the race was just a couple of paces from what I thought was the finish, so I slowed up. My friends grabbed me and pulled me a couple of paces across. Frustrating.
Paul, I've been running on the track this last week to not aggravate my hamstring, which running on inclines would do. I'll run maybe 6 or 7 miles on the track if I have to, but I'd prefer to run on the road. The track is nice because it's flat and you can zone out without having to watch for cars.