How do animals end up in remote areas?
I don't imagine a frog is going to go hopping over a hill to get to a marsh on the other side, is it?


Why not? In wet weather, the conditions would be just fine for an enterprising amphibian to go exploring, perhaps driven by predators or lack of available resources, not to mention Ro Siv's comment about wind-borne dispersal of animals. Birds of prey may also pick up small animals, fly away with them, then drop them for whatever reason.

You are also neglecting the fact that these organisms have existed for millions of years, and the landscape has changed tremendously in that time, with glacial periods interspersed with interglacials every 20-50,000 years or so. As glaciers melt, large lakes form, and as those dried the organisms were dispersed across the land into smaller bodies of water. Streams and rivers form, transporting aquatic life forms, and may dry out and form isolated ponds, or they may be blocked for any number of reasons (mudslides, earthquakes, volcanic eruptions, beaver dams, fallen trees, etc.). As erosion and landscape remodeling occur, all traces of the original body of water may be erased, except for the wide distribution of similar organisms.