There are surfaces that are a walker's dream. Britain is a land of sheep, and many surfaces are close cropped turf that is springy underfoot. In the North Carneddau I have seen wild horses cropping near a 2900 foot summit, and their grazing adds to the quality of the lovely, springy turf.
Yet there are more difficult surfaces. One time I was in County Leitrim, North West Ireland, trudging over the lower slopes of the Glenfarne Plateau, trying to find a route to the top. I struggled across the bog, my human body lumbering heavily as I tried to keep my balance. Then a mountain hare shot past me, shaming my human clumsiness as it sped lightly over the bog and disappeared into the distance. This showed me that I was in terrain that was not the natural home to humans. I belong further downhill, and the bog is unfriendly land into which I must venture as a visitor. Boggy land can sap your energy quicker than terra firma can.
I was in a land where the bog is merely difficult, but at times bogs can be dangerous. There are some areas where it is deep enough to sink into. One place that has some lethal bogs is Dartmoor in South West England. It also in has hidden mine shafts into which the unsuspecting can stumble. Whatever you do in places like this, stick to the paths.
In fact it is safer to stick to paths in all circumstances, as the paths have evolved through time to take the safest and easiest routes. An example is the tourist path up Ben Nevis, which is perfectly safe as long as you stick to it, but there are terrifying cliffs that threaten those who stray off route, and one dangerous place in them is Five Fingered Gulley, which has taken many lives. Do not stray along this path in the dark, and do not stray off it in daylight
Some mountains have stony sections that make for heavy walking. The Glydrs of North Wales have a name that means clutter; and below you see a section of Croagh Patrick, Ireland's holy mountain. It is not only steep and prone to mists, but has paths that are often rocky. There is no wrong in walking rocky paths, but realize that an ankle can be turned and that they can sap the energy if you walk on them.
Similarly, there are area covered with thick heather, which can look beautiful but can drain your energy if you try to walk for long distances across it. The thick heather of the Brecon Beacons is used for training by the SAS, the elite special forces unit. They have to run across it to build fitness. But these are ultimate fighting men. They have to handle terrain like this, you and I don't. Stick to the paths.