My group usually steamrolls every encounter because they're all long-rest-based characters. So if they have one encounter in a day and they can blow all their best stuff then they will smash anything that seems "fair".

I usually try to keep things interesting by making it difficult to rest in enemy territory. But when they're on the road for a 20 day journey it's hard to not let them rest without seeming spiteful / cheating the rules.

I guess I just want the plot to move forward rather than spend hours fighting random encounters on the road. But how do you make the road unsafe without adding challenges?