The Disney first aid stations see quite a bit of this. They are saying it's actually broken capillaries below the skin caused by the heat reflecting back up off the pavement. It doesn't effect everyone, but they have seen every type, age and size of person there is come in complaining about it. Broken capillaries will turn red, blotchy and for most folk, itch horribly. Treatment: elevate the legs and apply cool cloths. Prevent: use a non-pour clogging sunscreen (avoid those water/sweat proof ones which trap heat), wear white socks that cover the lower leg, light colored loose clothing to cover the skin such as capris.