Tomb of Annihilation says:
On the map of Chult, each hex measures 10 miles across. Characters moving at a normal pace can travel 1 hex per day on foot through coastal, jungle, mountain, swamp, or wasteland terrain. They can travel 2 hexes per day if they’re traveling by canoe on a river or lake...If characters move at a fast pace, the easiest way to deal with their progress is to roll a d4. On a roll of 3 or 4, they advance 1 additional hex that day...If characters set a slow pace, roll a d4. On a roll of 1 or 2, they advance 1 fewer hex that day (in other words, 1 hex by canoe or none by foot).
Instead of tracking movement by hexes, you can keep track of the actual distances covered (10 miles per day at a normal pace, 15 miles per day at a fast pace, or 9 miles per day at a slow pace), but this is likely to be more bother than it’s worth if the group switches pace from day to day.
One player proposed the 6th level Totem Warrior barbarian feature Aspect of the Beast, for Elk:
Whether mounted or on foot, your travel pace is doubled, as is the travel pace of up to ten companions while they’re within 60 feet of you and you’re not incapacitated.
Since "fast pace" gets 1.5 hex on average, and "slow pace" gets .5 hex on average: I'm not sure if I should double these values, give double hex movement with two d4 rolls, or double hex movement with still only one roll, or if there is some other way I haven't considered fully yet.