Distances to cover in a day

It depends
On the time of year.
Are your characters travelling across country or on well-maintained roads?

Traveller on foot 15-25 miles/day
Able-bodied rider in a hurry 30/40 mile/day
Average traveller with baggage 20/30 miles/day (from 'The Medieval Traveller - Ohler)

For children - slower
For cross country - slower
For bad weather - slower (think muddy roads and crossing rivers in spate)

You have to consider too whether they are covering this distance day after day or if this is a one or two day dash. It's harder to keep up the pace day afer day.

distance

It depends so much on teh person, and on the terrain, that it's hard to say. I've walked thirty miles per day many times. This is only ten hours walking at an average pace. Slightly below average pace for me. It actually took about eleven and a half hours each time because of three half-hour breaks.

A man in good physical shape, and who is used to walking, can cover fifty miles per day, and a horse can cover 100 miles per day. But you wouldn't want to do either unless it's an emergency. Neither of these distances is anywhere near a record, but they are the maximum most can reach, no matter how hard they try.

There are still 100 mile per day horse races held in some countries, and a good horse can do the job without dying.

But by and large, twenty miles per day is a fair walk for a man. This can be done without undue strain, assuming good terrain, and no heavy pack. Forty miles is about the limit for a horse carrying a rider.

Just a thought...

English recruiting sergeants in the 1640s only expected 8 miles a day from their recruits walking to barracks. This was due to their general level of fitness and the usual bad condition of the roads, tracks or footpaths.

Twenty miles per day is possible over roughish terrain and rough footpaths, (the Yorkshire Dales, actually), for fittish people, with packs, although that's pushing it a bit. I once met a couple of hikers in their late teens who were doing this, though most people would consider fifteen to be more reasonable. To keep it up day after day would be difficult. On a day after day basis, I would say ten miles per day.

Other factors: alone or a group, marching armies varied quite a bit in their rate of march. The old Roman rule is worth looking up. How long is the trip? Boy Scout troops sometimes attempt to cover a hundred miles in a day but of course are totally wiped afterwards. The American Army discovered that on very long marches the infantry arrived ahead of the cavalry. Transport animals impact the speed of march, oxen are slower then horses.
All said it takes effort and discipline for groups to consistently march more then ten miles a day and the fifteen mile days will be offset by the days spent crossing rivers and climbing passes.