There are a lot of towns that you will never get to if you just follow the main storyline and mission sequence. The Southern Shiverpeaks region is quite large. Of the eleven towns in the Southern Shiverpeaks, the storyline lead you to only four. There are others in Kryta, Maguuma, Northern Shiverpeaks, etc. too.
AW Lore explained this, but I can elaborate if needed.
Fissure of Woe and the Underworld are special high-level areas, accessible from the Temple of Ages in Kryta. To enter, the region you're in has to have 'favor', then you kneel at the appropriate statue and pay an entry fee of one platinum. They're special areas, with repeatable quests and better than average drops, for ascended characters only.
Sorrow's Furnace is accessible from Grenth's Footprint, an explorable area in the Southern Shiverpeaks. (Check
here for an old map: Deldrimor War Camp is the closest town, and it is south-west of the Granite Citadel and north-east of Camp Rankor.)
HoH is the Hall of Heroes'. It is PvP, not PvE. You have to win in Heroes' Ascent to get there, which is located on the Battle Isles.
You don't have to pay for a runner, though it is possible for SF. And there are quite a few hidden things out there to find. Exploring is fun.