Other answers have looked at longer-term implications such as days off, and food. I'm going to look at the impact of terrain. Your question states "rolling hills and trees" which describes a huge variety of terrains.
A recent hike went along a many terrains and trail conditions. Numbers are guestimates based on how fast I think we were travelling rather than actual measurements, but it should give you an idea how much it can change even over "rolling hills and trees":
- Well maintained trail was about 5 or 6 km/hr. It was gravelled, and not much different to a normal pavement...
- River flats without any trails (tussock, grasses, smallish stones) were crossed at about the same as a good trail, so about 5km/hr.
- Forest with 4-year-unmaintained trail was reasonably fast, probably 4km/hr due to occasional use of a machette.
- Forest with a marked but unmaintained trail (10+ years) was slower, probably 2km/hr. In places the trail had washed out, requiring significant amounts of bush-bashing. In some places the trail markers led to us taking more time because the ground or plants had changed.
- Mature forest without trails we crossed at about 1km/hr due to bogs, fallen trees and thickets. Lots of back-tracking was necessary.
- Young forest without trail (scrub about 1.5-2m high) we moved at maybe 0.25km/hr because there's only so fast you can swing a machete and in small scrub there's no elbow room to get a good swing.
- Bogs are impassable if they are more than a few meters across. They tend to stretch along shallow valleys and can require significant time to navigate around.
On this particular hike of about 7km, it took us 5 hours to go one way, and 3 to go back because we had already "broken" the trail. The whole hike was "flat" in that the end-to-end elevation was only 200m or so. However, we probably covered a kilometer or more in vertical elevation changes as we dropped down into little gullies, skirted around slips and so on.
Emperically, on this "flat" hike through forest, we were averaging about 2km/hr (1.2mph) across a variety of track conditions and terrain types. If you assume 7-8 hours walking, this about lines up with @elemtilas's answer looking at Lewis & Clark's expedition where they covered about 9 miles per day.
This was through New Zealand beech forest, which looks like this:
(Image from https://www.doc.govt.nz/nature/native-plants/beech-forest/ )
Some final notes:
- When bush-bashing through a mature forest, the slow part isn't cutting your way through the forest, it's all the back-tracking you have to do to dodge bogs, the detours to avoid fallen trees that are to big to climb, the washouts and sinkholes you have to walk around. The only exception is when going through young scrub, where it all about how fast you can cut, and cutting is extremely slow.
- The numbers given above are based on a single journey, and none of us are experienced off-track walkers, although we were all fit and had done a significant amount of on-track hiking across similar terrain. A person with more off-track experience could probably go faster because they could predict the location of bogs and washouts without having to see them.
- This was all in good weather. Recent rain would have made everything slippery and muddy and slowed the pace even more.