The lowering HR sounds to my newby self like your legs were wearing out, and you couldn't work hard enough to keep your HR up.
Not sure on the dizziness. I only get lightheaded well after a hard effort, if I'm resting with my legs up and then stand quickly. I get orthostatic hypotension then, and fainted from it once. That went over well on the home front! Hadn't been seeing it for quite awhile, but I am after this past weekend. It's entirely possible that eating has something to do with it for me, given I recently dropped a couple of pounds. Just to be lighter than AJ.
187.3 this morning. I've graduated from "Addiction" to "Pathos", or so I'm told.