I'll get into the second or third rep of a workout and my legs get very heavy, and have to slow way down. Eventually I cannot even run slowly and have to stop. It's accompanied by excessive sweating and chills/dizziness.
At first I assumed dehydration, but I've consumed at least 36 oz of water before the workout, never being thirsty or having dry mouth. I next thought it was anemia but i get over 100% daily iron each day.
It's very hot and humid where I live. However, I've run in these conditions before and I never had this problem. This has been going on for a few months now. Race times have continued to plummet over the summer. run about 70-80 MPW right now.