They are completely different sports, as stated already. Football is a contact sport with short bursts of speed, where as with soccer the players are constantly running. I played soccer and basketball on teams and running in basketball was a breeze after soccer season. I just recently played pick up football for the first time and did surprisingly well. I do not have a large frame to be very physical with larger players, but my strong point was lasting speed. Not only would I out-pace them, but I could still go the same pace after an hour of playing.
This is not to say Soccer players would be better at football than football players, football players are conditioned to move competitors out of the way and have more hard hitting. Where as in soccer you try to limit contact. Just different animals.