Long Rests

Life on the trail is arduous work. The adventurers will work 16 hours a day in order to move approximately 16 miles, and then take shifts on guard duty through the night. There are also the intrinsic dangers of the trail, poor weather being the most common. All of this, day after day, for months is incredibly tiring. In addition, driving the cattle constantly without giving them adequate time to graze affects their mass and stamina.
Therefore, while they are on the trail, the following house rule will be in effect:

Travel Weariness

After any day spent traveling for more than 4 hours, gaining the benefits of a long rest requires 24 hours of rest.

From a game and campaign perspective, the PCs will be spending many days (months actually) traveling to their destination and they may have only one or two encounters on any given day. This means they would have the benefits of a long rest (as described in the PHB) for every single encounter, meaning they can steamroll them with their most powerful abilities. This house rule allows for a more normal distribution of encounters.