Painful 5 miles

I originally planned on running this evening, but got a break during work and thought I'd get it done. I really didn't want to go and I felt if I didn't "just do it" I might talk myself out of it. I started off great, just over an 11 min pace for my first mile, then my right ankle started hurting. A lot. I tried for a while to keep running but eventually gave up and had to walk the rest of the way. Combine the ankle pain with the absolute energy drain from the heat and I just couldn't do it. I'm really going to have to either start doing mornings or continue with nights. Or, bleh, start doing the treadmill. I really, really dislike summer..