I hear ya. Martial arts can be a great "replacement," physically and mentally. Tackling people in football is fun, but it doesn't translate well to self defense and learning how to use your muscles efficiently. When I was in China, I saw 80 year-olds exercising in the park that were more coordinated than most teenagers today.
As far as intensity...it can be intense as he wants, depending upon the form....from something mellow like tai chi to "brutal" arts from Malaysia and the Phillipines, that make football look timid.
Have nice evening, sir.