I'm running a game with a couple of friends, one of which is a druid with a base speed of 8, and the party has hired some horses (speed 10) but almost lost one in combat as an enemy tried to escape on it. The druid then said, half jokingly, that he could just run to keep with them if he had lost the horse since running gives a +2 to speed, assuming it applies outside of combat at the DM's discretion.
It's got me curious since the rules generally cover walking, resting and what-not; are there any rules/penalties that govern, I guess you'd call it fatigue outside of combat? or are there any suggestions on house ruling something like it?
Scenarios could include: running a message from one village to another and back, which might take the party an entire day or two if you include rests but might only take half a day for the one character running, or running into an encounter i.e from village to cave to fight a bear, or being ambushed.