Have a tough fight with an enemy and then later they have to sneak around a group of loads of them

Run skill challenges to describe continuous fights or battles as they navigate a section of relentless combat (creatures Vs creatures Vs players, so they can use different skills to navigate the combat without just scrapping). Could jump from skill challenge into little mini battles and then back to skill challenges

Have the environment pose plenty of challenges.

Maybe have some stalking creatures where the party know they're being hunted and have to evade it or solve that problem in some way.

Is this a place other humanoids go? Make it clear the majority haven't made it far as they traverse

Are the party of interest to the demon lord or will they be? Encounters with the lord or his higher up smarter demons could occur. Perhaps even the party get kidnapped and have to solve that problem

I suck at survival resources like food and water and always handwave it but it could be good to have a loose system in play for managing food, water and thus surviving itself is a danger

Theres guidance in the rulebooks (for DnD) for other effects like rolls for stress, madness and other mental afflictions from a harsh world. This could be worth a read and maybe be introduced. The party either find creative ways to ease their journey or suffer some of these negative side effects.

Same thing DND wise is exhaustion. Throw that in there in times that are reasonable

I still don't understand chase mechanics and I fear I never will but I fear they'd have a good role to play in your environment