Professional sports coaches are almost always college players that couldn't play professionally. They get an entry level coaching job working for peanuts and work their way up over many, many years. They basically learn how to coach on the job, UNDER THE SUPERVISION OF MORE EXPERIENCED COACHES. They aren't running their own programs and learning as they go.
That's why you see "lineages" of head coaches. At one time, every head coach in the NFL had been an assistant under Tom Landry. In the 80s and early 90s, almost all the head coaches had been assistants at SF. Up until a couple of years ago, about 1/2 off all the head coaches were at some time assistants under Jimmy Johnson.
And I can guarantee you none of them had 5 years or less of training to do their jobs.
For example:
Bill Cowher began his NFL career as a free-agent linebacker with the Philadelphia Eagles in 1979, and then signed with the Cleveland Browns the following year. Cowher played three seasons (1980-82) in Cleveland before being traded back to the Eagles, where he played two more years (1983-84). Cowher began his coaching career in 1985 at age 28 under Marty Schottenheimer with the Browns. He was the Browns’ special teams coach in 1985-86 and secondary coach in 1987-88 before following Schottenheimer to the Kansas City Chiefs in 1989 as defensive coordinator. He became head coach of the Steelers in 1992.
That's 17 years of playing/training (including college) before he became a head coach.
Belichick was a center/tight end at Wesleyan 1971-74. He launched his career in 1975 as a special assistant with the Baltimore Colts, then became an assistant special teams coach with Detroit (1976-77) and Denver (1978). In 1979, he joined the New York Giants as the special teams coach, and by 1981 he was also working with the linebackers. In 1985, he was named defensive coordinator and contributed to the Giants winning Super Bowl titles in 1986 and again in 1990. Following Super Bowl XXV, Belichick was named head coach of the Cleveland Browns in 1991, becoming the youngest head coach in the NFL at age 37.
That's 20 years of playing/coaching experience before getting a head coaching job...
Sure, some guys get there in less time. But they usually don't stick around very long...