Lewis and Harris is the main islands of the Outer Hebrides. So, it's not two islands. Lewis is the northern part, pretty flat, and navigating over the moor requires map and compass. Harris is the mountainous south part of the island. There is no marked long distance trail on the island. But many shorter walks fan out into wild areas. Once you get all the way to this island, you will probably like to hop to the other islands too, each of which is very different in character.