You do'nt need to do a thing to make a forced march. it extends your generals' movement plus his troops range by up to 6 inches on a 17 inch monitor. retinues are a bit preprogrammed they have them 1 turn before you get a notification that they have them(at least for forced march). So if you have a general walking the redline it means he has a force march retinue or trait, and it will show you the message next turn. If you are always moving your general in the green zone, I do'nt think there is much chance he'll pick this up but if he constantly has to go places that takes more than 1 turn often this will be picked up.