Training camp begins in July. The "pre-season' games start in early August (they are 4 practice games that do not count)
Regular season games start in early September and last sixteen weeks, with one week off for each team sometime during that span, which runs to late December. Then there's three weeks of Playoff games to determine who makes it to the big dance. (early Feb)
The NFL Players Union won't let the season get any longer, much to the owners chagrin.
The physicality of the game has a lot to do with it, yes. It's a nasty game with many injuries.
Yeah can imagine after hearing some of the injuries over the years. Guess alright if you are a big contract player, guess the money may compensate a career ending injury.