http://en.wikipedia.org/wiki/Endurance_riding
Winning riders complete 100-mile (160 km) rides in 10-12 hours.
http://stevengalen.com/ultra/world_ultra_records.html
100 mile 11:28:03 Oleg Kharitonov (GBR) 2002
The human time is for track running (it is a bit over 12 hours for "road"), while the horse times are for "trails". Horses also have to carry a pesky human (with at least 75kg of ballast) for the whole distance.

[OTOH, there was a famous "race around the bases" of 360 feet involving Hans Lobert back in 1913, who got squeezed in the home stretch to lose stride and lost by a nose -- admittedly, he had the inside line the whole way as per the rules].