Say a dungeon is 10 squares away. You roll random encounters and that takes up 2-3 hours of play because you want the feel of journeying through the wilderness.

They get to the dungeon but only make it through 3 rooms before the session ends. They end back in town like every other session. A week passes and a player wants to finish that dungeon. So he gets a group of different players together and they go the same 10 hexes, face 2-3 random encounters, get to the dungeon and run out of time again before they can search the final 2 rooms.

How do you deal with this issue? If every session ends back at the start how are players supposed to adventure further and further away and not spend the whole session time battling the random encounters getting there?

I've been a DM for a long time. I'm good at summarizing travel when it isn't important. But West Marches style gameplay is centered around venturing into the dangerous wilderness. How can I still give that experience of players running the risk of getting lost, battling wandering monsters, and eventually getting to their destination if they have to start over from hex 1 if they didn't have enough time to finish the content? They'd get stuck in a pattern of having to spend all their time getting there and then stopping. How do I fix this? I want to understand before I attempt a West Marches style game.