To give you a better feel for why it is challenging to determine speed from video, consider the screen capture below (click on the image for a larger view).
You can see the NFL's speedometer showing Manning running at 17 mph. There is, of course, error associated with that number. What I really want you to see is the orange arrow I put on Manning. That shows approximately the direction of his velocity vector as he ran past the 21-yard line. My modelling of the run, therefore, began after he squared his shoulders and got moving more parallel to the sideline.
I found a maximum speed of a little more than 16 mph (26 kph), and I estimated an error of no more than 10%. Given what I had to work with, I would say the NFL's speedometer was reasonably accurate. But as I mentioned above, an overhead view would have allowed me to determine his velocity vector instead of just the component of that vector parallel to the sideline.
Manning is listed at 6' 5" (1.96 m) tall and he's been a great athlete most of his life. Athleticism and a long stride length help with speed (just ask Usain Bolt, whose height is about the same as Manning's). Still, hitting 16 mph - 17 mph certainly isn't bad for a guy nearing 40 years old!