I've found that running in direct sun I stay cooler with a compression shirt w/sleeves than w/o. I never run shirtless(my extreme albinoness might make cars swirve into the blinding light)
Judging by your run time and comment on the last 3 miles I would venture a guess that you might have a pacing problem. I run a 5k right around the 20min. mark and a 10k in 41min, I notice a little drop toward the end of the 10k but try to make a mental effort to keep my per mile times within a 15-20 second range.(Might be worth trying for your 1/2m)