Professor Peter Weyand, Southern Methodist University (Texas), known for his expertise in terrestrial locomotion and human and animal performance says that humans would soon have the ability to modify and greatly enhance muscle fibre strength. This would enable speeds of 45 miles per hour and 5 seconds times for 100 meters.
The fast four-legged runners or quadrupeds do seem to be advantaged versus bipeds in terms of the mechanics allowed by their anatomy. So to go even faster would require people to successfully adopt the running mechanics of four legged animals (running on hands and feet).
DARPA is also spending 3 billion to enhance strength and endurance. So endurance enhancements combined with the muscle speed enhancements could allow sprinting for an entire mile run. This would mean 80 seconds to run one mile. 1 minute and 20 seconds. It could also mean about 40 minutes to run a marathon.