The Pronghorned Antelope.
It's sprint speed is in the 60+mph range, putting it possibly second behind the cheetah (which is on a different continent, anyway). But, it can sustain relatively high speeds for much longer than the cheetah: it can run over 40mph for 5 miles. That is an 8k in 7:30ish, which should podium, even among LR posters.
It has very lightweight bones in its legs, and enlarged windpipe, built-in shock absorbers in its hooves, and something in its nose that cools the air it breathes to keep it from overheating.
There. You learned something today :-)