Tom
i greatly suspect that most of it is Physcological
i've only run 4 HM's in 6 months, been off 3 months with Prostate problems, and for first time in my life of running ( 40 years of it ) my GP advises me to run with caution especially in any heat
one day i'm up for it and feel like going for it, another day i dont -
sport , ( in our case running ) is first meant to be fun and enjoyable, to keep fit and feel good about ones self - it's ok for me to talk i know, - i'll bounce back shortly and vast expereince will be on my side
but for some of lesser expereince, the body and mind becomes lathargic, and if prepartion before a run isnt right ( IE fluid ) any runs with just get harder
i would suggest until momentum returns to run little and often and think positive about the good things- forget chasing PB's for a while, get that feel good factor back first
best wishes always
Mick