Running in the heat and running at altitude are remarkably similar. In both cases, not as much oxygen is available for the running muscles as a nice cool day on the 380 meter long track at Stanford. (It's a joke, it's a 400 meter standard track, but people run so fast there, it seems like it's short.) However the causes are very different.
At high altitudes, your lungs strain to take in more air, which really means you try to take in more oxygen. Since there is ultimately less oxygen circulating in your body, it gets allocated based on the priorities of your body, namely your brain and you heart get their fair share, but your leg muscles get less than normal.
In hot weather, you may have plenty of oxygen, but your body prioritizes sending blood to your skin to cool you down through sweat. Now, the plasma is really what is doing the work of sweat, most people don't sweat red blood cells. However, the effect is essentially the same, less oxygen is getting to the legs for the running muscles.
I suppose, you can call this just a theory of mine, but it makes a lot of sense to me. Both running conditions are slower and more tiring compared to 60 Fahrenheit weather at sea level.