> I find it difficult to imagine unless you corner the animal and have enough manpower to not be run over.It would be easy to imagine, if you had experience of collapsing from fatigue. Try to get on the roof of a skyscraper by stairs, you'll get all the needed experience. For more realism, you could run for a several floors, then rest for a few minutes, and to repeat it as needed. For even more realism find a person who is better than you at marathons, give him/her a stick and tell him/her to move slowly up by the stairs (1-2 steps per second, I believe, will do) and to beat you with the stick when he catches up with you. At some point you will prefer stick to moving further. It could be done without stairs on the plains, but it would take more time, up to a few hours.
I couldn't give you experience, but I could give you some theory. Moving animal needs to absorb oxygen, to oxidize glucose into ATP. If animal tries to move faster than some limit based on the ability to absorb and utilize oxygen, then some of glucose metabolizes only halfway to H2O+CO2 and lactic acid. Muscles start to accumulate lactic acid. If concentration of lactic acid in muscles is high enough, animal feels it painful to strain muscles. The more concentration is the less strain is needed to feel pain, the higher level of pain at the same strain level. At some point even the slightest straining feels painful. Cells in muscles could die from too much acidity, but it hard to do because of pain. (Though a heart could be damaged this way, for this one needs to force his heart to work at ~180+ beats per minute for a ~10+ minutes, it doesn't feels like pain in the heart, so be careful when pushing your body to its limits).
To get rid of lactic acid animal needs oxygen. Muscles could metabolize lactic acid to get energy, or the liver could do it. But in any case it needs oxygen, animal need to stop and to rest, or at least to run at lesser power, to have enough spare oxygen in a bloodstream.
If animal have no time to stop and rest for long enough time, then lactic acid concentration continue to grow. It feels like overwhelming fatigue, pain in muscles, and all you want is to take a break. If you try to work your muscles through it, at some point they just stop working. But it is hard to get to this point, because of a pain and a loss of motivation to continue.