Looking at the Olympic records; the 100m is 9.69sec with an average speed of 10.32m/s, the 200m is 19.3 with 10.36m/s and the 400 is 43.18 with 9.26m/s.
It would seem that a (the best!) human can run flat out for 20sec, but certainly not 43sec (speed falls off a lot). I suspect around 20sec is the limit.
I beg to differ. The reason that the 400m is slower is not that they're not running full pelt but that they're not running as fast (plus corners slow you down). Take a 400m runner and pop them on a 100m track and they're not going to run as fast as the 100m sprinters.
And again this thread fails...
And again this thread fails...