Unmechanized travel: Afoot
Someone asked me recently, in regards to their story (not here, by the way) "How long would it take to walk from X to Y? And how long would it take to march, say, a company of soldiers from X to Y?"
While there are general guidelines (which I'll get into in a moment) there's also one Rule: and that is, the story trumps all. The story needs to work *as a story*, and if you need to rearrange the geography to suit the story...do it. (C.J. Cherryh says "Never draw the map first.")
You can find out a lot of useful information on foot travel by a) doing it, b) looking up typical times for walkers to cover the kind of terrain in the kind of weather you've provided in the story, c) reading a lot of military history (in which you'll find that some general staff, despite knowing how fast their army really could march, based their plans on having it march much faster than it could, with unfortunate results.) You can also watch videos of people walking in various terrain, with various loads--for instance, there's an available video about the Appalachian Trail, in the US (runs over a thousand miles from Georgia to Maine along the spine of those mountains). I've seen videos of people hiking elsewhere in the world, too. For some stories you do have to remove the modern lightweight gear they use and replace it with something appropriate to your story.
Doing it--at least some--is good because it gives you the body feel of walking on different terrains and slopes, with varying loads. It's altogether different to go up a steep mountain trail in a dry area where you have to carry your own water and you've got two gallons as well as your other gear--and coming down with nearly-empty water jugs, less food, etc. Every surface is different: wet snow, dry snow, deep snow (wet or dry), mud of various depths and consistencies, wet sand, dry loose sand, rocky, loose rocky, rounded gravel, shallow water, deeper water, close-cropped turf, knee-high grass, waist-high grass, etc., etc. Those of you in the UK are lucky because you have so many different kinds of terrain and walking surface close enough together to make getting that experience easy.
For story purposes, we care about what makes the written version believable to those who have done it or something similar. To do that, we have work with the following factors and their interactions--the person walking, the surface walked on, the terrain (steepness), the weather, the length of the journey (a one-day sprint or a month-long endurance trial?) and the load carried (its weight, its shape and size, whether alive and wiggly like a baby or inert like a rock.)
The Walker: fitness, nutritional status, hydration status, health status, all affect the rate of travel. Fit are faster than unfit. Walkers who have adequate nutrition and water available travel faster than the malnourished. Walkers who are healthy are faster than those who are sick or injured. Age is also a factor: a given individual will be slower at 60 than at 25, and is more likely to be slowed by environmental factors such as temperature. Fit individuals used to walking rapidly and lightly loaded may make 5 mph on level firm surfaces when no other factor intervenes. However, they are unlikely to walk 5 mph for 12 hours and achieve a 60 mile day. Although humans can work longer at a stretch than, say, horses, they're still animals and they still have limits. One of the limits is skin strength on the parts of the feet that contact socks and shoes. (I once did a long hike--6 days in remote territory--and acquired serious blisters on both big toes. I could walk my normal speed on firm ground, but in the deep sand we encountered repeatedly, it slowed me WAY down.)
The Surface: The surface determines both the stress to joints and bones, and the energy expenditure: the easier and less stressful it is, the faster the Walker can walk. Resilient surfaces are easier to walk on--require less energy per step and stress joints and bones less. Short-cropped turf over a soil with a little elasticity would be an example of such a natural surface. Firm (but not totally unyielding) smooth (but not slippery) surfaces are next in ease. Hard surfaces (rock, concrete pavement) are tough on feet, joints, bones, but offer no other impediment. Soft surfaces exact an energy cost (some of the energy is lost in the subsidence of the surface) and sort surfaces that cling (like mud) exact a higher energy cost. The deeper and most "sucking" the mud, the slower your Walker. Uneven surfaces (rocky trails) also require more energy and prevent the rhythmic stride that is most efficient--same with twisty-turny trails. Loose surfaces (gravel, sand, other loose rock) require more effort and thus slow walkers down. On a slope, loose surfaces can shift downward, causing walkers to climb multiple of the slope to get to the top. (Walking on scree: not fun.)
The Terrain: Terrain here includes all physical properties of the route, including vegetation, but mostly concerns slope--uphill, downhill, how steep--and watercourses to cross--how deep, how fast. Terrain partly determines surface (swamps have sticky mud) but not completely, as you can also have mud on level uplands. Terrain determines whether a route can be straight or must twist and turn around obstacles. The steeper the uphill slope, the slower the walker. It's possible to faster downhill, at the risk of injury from falls and quadriceps cramps of immobilizing intensity. (I once ran down a mountain, with maybe 30 pounds left in my pack after a week on the mountain, because I could and thought all those "never run down a mountain" warnings were silly. Why not? Trail was dry; I'd been on it both ways before. Safe as houses. Steep, switchbacks (plant walking stick and swing around) enormous fun. Then. A few hours later, trying to stand up? Not so fun.)
The Weather. People do sustained walking fastest in weather that is comfortable while working at that level--when they aren't too hot or too cold, aren't being pushed by a strong headwind or crosswind, aren't being pelted with rain, hail, sleet, or snow. People not conditioned to cold climates slow more as the temperature drops than those who are conditioned; the same is true in hot climates. Older walkers, in general, have less internal temperature stability than younger ones and thus are more likely to slow down (and get in trouble) as temperatures soar or drop.
Length of Journey. If it's possible (however hard) to make the distance in one day, people can walk faster/farther for one day than they can sustain day after day. Long journeys reduce the average walking speed and the average ground covered/day.
Load Carried. Additional weight requires more energy to transport and thus generally slows speed. Although someone may become conditioned to an increase in body weight and continue to walk at the same speed, putting a pack on that person's back and loading it down will slow the walker. The size and shape of the load, and its nature (wiggly v. inert) and the equipment used to attach it to the walker, all affect how much the weight slows him/her down. Research done back in 20th century (and published in a major science journal) proved that the energy efficiency of African women carrying loads on their heads exceeded that of men carrying equivalent loads on their backs, because of the effect of load placement on center of gravity. (Yes, it does long-term damage to neck vertebrae...but overall this upright, centered load allows the women to carry bigger loads efficiently.)
Another factor is group size. The larger the group, the slower it moves and the more likely it is to have laggards (walking behind a large group on a dry day means dust in your face all the time--it's natural to back off hoping for cleaner air. On a rainy day it means the first rank's walking on grass, the fifth rank's walking on squshy grass, the end of the formation's shin-deep in muck.) Even on a paved road, a formation of may stretch along the road for a mile or more. By the time the front rank's where you want them, the last still has a mile to go. Military history is full of the problems of moving tens to hundreds to thousands to tens of thousands of troops around terrain--well worth reading if you're writing stories that need to move troops.
All these factors have story utility. If you want to slow your party down (to be caught by pursuers, to not reach the next town in time to prevent something), you have many ways to do it. If your geography is set (you're using real places) and you want to speed them up, you'll have to find them a straighter route (thus shorter), and ideal surface and weather and lighten their load. Figure on an average of 3 mph in undulating terrain if your characters are carrying a medium load and are fit. A single very fit person can average more, but nobody's going to average 10 mph, no matter what. If your geography is not set yet, use travel time, not formal distance, for the space between city and city. ("Berryville? It's a half-day's walk past that oak wood there...take the right fork, or you'll be in the river...")
(Coming soon to a forum near you: animal-powered travel.)