On one trail I can run 3 miles almost continuously, with small 20 second breaks. However, when I get on the 8 mile trail, at about 4 or 5 miles into the run, my legs start to hurt very abruptly and feel really heavy. My cardio is good enough to keep going, but I'm unable to move my legs faster. I have to walk frustrated, sometimes limping the rest of the trail, when I could have ran all the way. I'm probably missing something. Why legs die like that? and is there an exercise of some sort or a diet to improve this?