I went out on Sunday for my long run intending to do 1hr 40 minutes slowly. That's 10 minutes longer than I've run before - I managed a hour and a half last Sunday with no problems at all. Anyway, I had to stop at 1:20 - I was just exhausted. My legs just wouldn't go any more. When I stopped my legs had a kind of tired ache for ages afterwards and I generally felt really washed out.
The night before I'd been out for a very nice meal, at which I drank quite a lot. But I've drunk more before a run before without any ill effects. One factor may be that I started the run at 0730 which is earlier than I've ever done it before so I didn't have time for breakfast - I just grabbed a banana instead. I drank lots of water before I left and I drank lucozade sport as I ran, so I don't think it was dehydration. One think I did notice was that I had a very elevated heart rate (65 vs the usual 47-53) first thing in the morning.
HR this morning was 55 - so getting back towards the normal range. I wonder what was going on.