Fox said that in last year's pre-season Manning had a lot of work to do but has been firing on all cylinders so far at the Broncos' training camp.
"[What stands out is] just how much further ahead Peyton is," Fox told NFL.com.
"A year ago this time, we were trying to get him to relearn how to throw. He's way more comfortable, way more ready to play."
Manning threw 4,659 from 400 completions in his first season at the Broncos last year.