In recent weeks, the day after a long run (13km+) I have felt decidedly "weird"... sort of tired and a bit spaced out. The run itself feels tiring but it's not a case of bursting lungs or legs feeling like they're on fire, just tiring. I suspect I just haven't got enough energy in the tank first thing in the morning for it not to have a delayed effect.
So when I start my HM training next month I am going to start taking an energy gel out with me for runs over 80mins. I've gone for the SiS selection pack (20gels, 4 lots of 5 different flavours)... will see how it goes and report back.