long ultramarathon. But I am skeptical if it correct.
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Below is AI answer
The world record for the longest distance run by a horse is 100 miles, set by Jayhal Shazal, an Arabian gelding, in 5 hours, 45 minutes, and 44 seconds. The record was set on March 20, 2010, during a race with rider Yousuf Ahmad Al Beloushi.