Football players play maybe 20 games a year (depending on the playoffs). Hockey players play 4 times that number.
Hockey players don't get "time outs." If they get injured, they'd better get the heck out of the way or they'll get run over.
Hockey players come "off the field" by leaping over the boards. Their teammates "take the field" by simultaneously crossing the boards going the other way. The game doesn't stop so you can do a line change.
Periods are 20 minutes long, and a team is moving almost the entire time. Activity stops for a face off, but that might take about 3 minutes, tops.
Football players are a tough bunch -- I certainly couldn't do their job. But hockey players have a much more grueling time. I skated with a bunch of them for 15 minutes many years ago, and thought I would die after the first 5. Not a job for the faint-hearted.
Played some games in the local beer league both with and against former and current minor leaguers (mostly ECHL and AHL types, though I regularly play against a guy who had a cup o' coffee with the Caps back in '79... he still throws elbows in the corner even at 54 y.o.!). Those guys, guys who couldn't even make it to the "real show," are absolutly amazing. Fast, strong, and quick! Depresses me every time I think about how fast they skate around me...