One of the things you really need to factor in is the weather, and how quickly that will mess with your plans. It's quite easy to have 15-20 mile visibility, and 20 minutes later be enveloped in mist thick enough to cut down sight to <20 yards. When that happens, unless you have a really good, current and detailed map, and the navigation skills to use it that moorland becomes dangerous to traverse. Lacking obvious features, criss crossed with waterways, bogs, and pits of mud, broken and undulating rock gardens and slopes of shifting scree, it's easy to get turned around, spend hours blundering about looking for a good path and suddenly find yourself walking in circles. Add in the dangers of toxic spills, the odd wandering nutters, cultists and para-critters, and it's very easy to cut down that 15-20 mile per day rate of advance, to 1-5 - and then the moorland becomes quite big enough.
There's a reason that the British Army uses places like
Dartmoor, the
Brecon Beacons, the
Yorkshire Dales and
The Cairngorms (Scottish Highlands) to train their units.
Despite being only 20-30 miles from fairly major cities in some cases, it's possible to become very isolated, to feel very remote and to find navigation very tough in these areas.
As an example, I go walking in the Yorkshire Dales a fair bit - have a look at
this photo album.
In some of the photos you can see for 15-20 miles, and may only be able to see a single farmhouse. In others you're looking at a 500m climb on 2/3 sides, and a 500m drop down a gulley on the others - all over undeveloped terrain. If you're curious,
this is the approx location of most of these photos.
If you're right in the centre of Scotland, then you're not more than 50-60 miles from the coast. Given the nature of the pollution of the north sea, and the freak weather that's now common in the SR era, that means you're very likely to get hit with frequent storms, and all sorts of other nasty stuff, with little to no warning.
If you're hitting your players with critters, magical threats and cultists already, adding in a bit of deceptive terrain, a few freak storms, some good quality acid rain and locals that have an inate distrust of anyone not born within 2 miles and you can make them have a *horrible* time.