There is no feeling like a thigh cramping mid stride on a long run. I can tell you that it feels like being shot in the leg, or to try and fight it off by stretching then continuing running concentrating on trying to run with 'relaxed' muscles. I can tell you that it is possible to run after pulling up in pain, but until you do it yourself it's hard to believe.
So far this year I've pushed myself harder than ever. I've never worried that my body would break down, I've remained confident that I can push through because I'm being smart and prepared with what I'm doing. My biggest barriers are mental. I've ridden over 500 miles this year yet my bike hasn't moved an inch. I've spent hours spinning and going nowhere. Every day I wake up and have to bike, that represents the hardest thing I have to do that day whether it's a quick 15 mile spin or 60 miles. It's not because my butt's going to hurt or my calf muscles are sore, it's because I know I'm going to get bored. I can't tell you how painful it is to sit in the aero position and watch the sweat beads on my arms grow until they pool together enough to have gravity pull them off my arm on to my towel. Time on my bike is my worst enemy. I've cut rides short because I couldn't take it anymore. But finishing those rides give me the feeling of my biggest training accomplishments.
Some times I have to remind myself that every mental victory spent in training gets deposited in my race bank. My body may break down, I may cramp, I may get a flat tire but having that race bank full come race day means that none of things that may happen during the race are going to prevent me from finishing because I've over come in training. I've passed those tests and I haven't backed down. I can't plan for everything and I can't experience everything that may happen during a race but if I make it a point to finish what I start in training, it won't matter.
