In the info tab there should be a stat telling you that quick march will last for X more minutes. X should start at 300 (I think?) and count down from when you started the march. 5 hours at double speed, basically. After that you're exhausted and move at half speed for a while. I don't remember how long exactly but it's longer than 5 hours.
If somebody's chasing you, you quick march to the nearest fief to escape. If you do it for no reason and you're left dragging your feet across a big field you're vulnerable because pretty much anyone will be able to catch and attack you. It's not used only to escape, but consider the advantages and disadvantages when using it.