Usain Bolt ran the 100 meter dash in 9.58 seconds, which places his average running speed at 23.35 miles per hour. The IAAF conducted a study on Bolt's record setting record setting run in 2009 and found that he achieved a foot speed of 27.44 miles per hour in that run.
It also appears that Usain Bolt can sustain his average speed at the 200 meter dash, which he was able to run in 19.19 seconds. His average running speed for this event is at 23.31 miles per hour.
For sustained long runs, champion long distance runner Wilson Kipsang Kiprotich ran the 26.2-mile full marathon event in 2 hours, 3 minutes and 23 seconds in Berlin in September 2013. This means that Kiprotich maintained a running speed of 12.7 miles per hour for 2 hours.
A study conducted by the Southern Methodist University shows the variables that places the limits on human running speeds. This same study concluded that it is possible to increase human foot speeds from the nearly 28 mph achieved by Bolt to 35 to 45 mph.