I was thinking about overland travel yesterday. I use the following overland travel rates in Blood & Treasure, which I lifted from Col. Sir Garnet J. Wolseley’s excellent The Soldier’s Pocket-Book for Field Service, based upon his military service during the 19th century.
The rates are as follows (miles per day):
Ass/Donkey - 16
Camel - 20
Dog - 10 (i.e. dog sled)
Elephant - 18
Griffon - 6 on foot / 18 on the wing (yeah, a few of these weren't from Col. Wolseley's book)
Hippogriff - 16 on foot / 32 on the wing
Horse - 16
Humans - 6 in a large group, 12 in a small group
Llama - 15
Mule - 16
Ox - 5
Pegasus - 16 on foot / 36 on the wing
Reindeer (team) - 75
To make it a little more organic, though, you could randomize it. In essence, let the adventurers roll a number of D6 per day to see how much progress they make each day. To keep it simple, divide those numbers above by 3.5 to find out how many dice to roll, rounding down. If the party has a druid or ranger with it, or a native guide, let them roll an extra D6. If traveling through especially tough terrain, roll D4's instead.
So, if traveling by donkeys, a group rolls 4d6 to see how many miles they cover in a day.
Another way to go would be to assume on major problem per day (if you've ever done a family road trip, you know this is probably generous) that the adventurers have to solve, usually through wise preparation. If they don't, they lose 1d6 miles of travel (maybe more) on that day. Some problems could include:
Monster Attack (can be solved by surprising the monsters and running away)
Heat Exhaustion (can be solved by wearing the proper clothing, drinking a double ration of water, etc.)
Injured Animal (can be solved by having extra mounts or pack animals, or casting the appropriate spells)
Broken Wagon (assuming wagons are being used - boy, would they slow you down - could be solved by having the proper tools and replacement parts or being able to cast spells like make whole or mending)
Illness (random minor complaint that can be solved with the proper spells - cure disease, purify food and water - assume it strikes at least half the party, maybe the half with the lowest Constitution scores)
You get the idea. This would reward smart planners and maybe provide some color to the journey.