The way I understand it is that hell is a different world to explore. You'll get there somehow, and you can get back, but you can also just explore hell for however long you want. Build a house, tame the lava fields, whatever.
Hell will be huge, but it will also let us travel even larger surface distances while we're there. The flyer says "portal in", which could be a portal that we build, or a portal that we can discover in-game. Either way, I'm sure it'll all be very dynamic from what I've seen so far.