I have created a table which I use as a guide for travelling speeds in my medieval-esque world. There are many factors which have an effect; not just the geography but the time of year, weather, and most importantly the size of the group. One thing worth noting is the very common mistake of thinking that being mounted improves travelling speed. It doesn't, over the long run. Horses can't walk much faster than humans; their main advantage is the amount of weight they can carry. Of course they can also travel much faster for very short periods of time, but these are very short. Most horses would be utterly exhausted after galloping for more than 5-10 minutes.

Here's my guide, km's per day:

SINGLE PERSON
On Foot - 35km (paved road) - 35km (open ground) - 20km (broken ground)
Mounted - 35km - 35km - 20km
Forced March - 70km - 70km - 40km

SMALL PARTY (up to 50)
On Foot - 30km (paved road) - 20km (open ground) - 15km (broken ground)
Mounted - 35km - 35km - 15km
Forced - 60km - 45km - 22km

LARGE PARTY (50-200)
On Foot - 30km (paved road) - 20km (open ground) - 12km (broken ground)
Mounted - 35km - 30km - 12km
Forced - 50km - 40km - 16km

SMALL ARMY (200-2000)
On Foot - 28km (paved road) - 22km (open ground) - N/A (broken ground)
Mounted - 28km - 22km - N/A
Forced - 40km - 35km - N/A

ARMY (2000-5000)
On Foot - 26km (paved road) - 12km (open ground) - N/A (broken ground)
Mounted - 26km - 12km - N/A
Forced - N/A - N/A - N/A

LARGE ARMY (5000+)
(here the determining factor is the length of the column, thus the width of the clear route is more relevant)
Wide Path (4 across) - 16km
Narrow Path (2 across) - 8km

For a group larger than about 10,000 the actual travelling speed will have to be determined on a case by case basis due to the complexities of composition, and the length of the column.

In addition to the above, I use the following multipliers to take into account variables. The guide above assumes good conditions on a summer day. For the factors below, the multiplier is a general guide, but obviously depending on the size and composition of the group, many of these variables would render travel impossible.

Desert - X 0.7
Rain - X 0.8
Storm (heavy rain) - X 0.5
Snow (on ground) - X 0.2
Blizzard (snow falling) - X 0.1
Swamp - X 0.1
Forest (light) - X 0.40
Forest (dense) - X 0.2
Winter - X 0.5
Autumn - X 0.75
Spring - X 0.75

You can see how these quickly affect travelling speed. For example a small party can nicely make 30km a day on good open country in summer. But in winter, through a light forest on broken ground in the middle of a Blizzard? Suddenly you're down to 300m a day!