Games

I think you can get from Capital Knot City to the Western Distribution Center without going over mountains. You have to take a long route North around the mountain.

There will be plenty of places you won't be able to bring the bike, though. As new structures become available to you, you'll be able to develop infrastructure to open up bike routes. Don't think of the bike as an upgrade to loping around on foot. It's infrastructure that needs to be supported.

I left my bike at the distribution centre and walked. But my partner actually made that drive. I still think walking was faster though. But it doesn't matter that much... You'll be out of the tutorial area soon. My advice is to just go. You'll get things that can traverse faster than a bike later in the game, and you can come back and 5 star things in that area later.

Also, just a tiny bit later in the game you'll have bikes coming out of your ears. It's fine to just abandon them when they're no longer convenient.

Edit: Also, there's a mechanic in the game that makes a path over your most travelled routes. So it gets easier the next time around.

Munrock already nailed it pretty succinctly but yeah, prepare to still be walking everywhere as vehicles aren't suited for the terrain largely. You'll get a lot more use out of finding other players vehicles in the wild to use when the opportunity strikes.

You'll eventually be able to build roads, but that is very much an optional/end-game task.

You can build makeshift roads with ladders over the rougher spots. Drive till you get stuck, lay down some ladders, drive across, pick up the ladders, repeat.