The key to a good run is first blocking up front, then some good speed on your running back. If you play a weak team (like one of the "FCS" teams), it's generally easier to find a hole and break through for big yardage.
Sometimes in open field, a quick tap of the B button to spin past a defender can help pick up lots of extra yards.
The Achievement will pop at the end of the play in which you rush for over 20 yards. Option plays are acceptable for the purposes of this Achievement. Note that it cannot be unlocked on a 20+ yard run in Road to Glory mode.