Overland travel uses a pace. At a normal pace a party covers about 3 miles per hour or 24 miles per day across 8 hours of travel. A fast pace is 4 mph / 30 miles a day, but you take a −5 penalty to passive Perception, making you easier to ambush. A slow pace is 2 mph / 18 miles a day and lets the party move stealthily.
A standard travel day is 8 hours. Pushing past that is a forced march: after each extra hour you make a Constitution saving throw (DC 10, +1 per hour past 8) or gain a level of exhaustion. Difficult terrain — dense forest, deep snow, steep hills — halves your speed, cutting the day’s distance in half.
Mounts and vehicles change the math: a riding horse is faster over short bursts, while wagons are limited to roads. For a long journey, multiply your daily distance by the number of travel days and add time for rests, weather, and obstacles.