I run. 77 degrees Fahrenheit, is awesome 2 run in. Go for it! Remember to hydrate, sun protect, & don't over do it. A half hour to 45 minute run is good. Also it's not about running fast, but maintaing a steady pace. Take care when running on trails, slick areas, and make sure to check your shoes every other month or so. Enjoy your workout!!
If it's 77 degrees outside and you were running, then you may faint and feel very weak or to the extent you will collapse, because of the too much perspiration that you may experience. Heat can cause heavy perspiration that makes your body drained of water.