I always find that if I keep my HR down below lets say 155 then I feel loads better after the run and also feel like I could carry on. Much above 155 and that isn't the case any more 
I always try and maintain below 150 and have an alert set to 145 or thereabouts, so I know to ease off a bit. It does take a bit of thought to follow HR rather than just going for it. I did find that the times weren't spectacularly different either.

I always try and maintain below 150 and have an alert set to 145 or thereabouts, so I know to ease off a bit. It does take a bit of thought to follow HR rather than just going for it. I did find that the times weren't spectacularly different either.