The characters have to have limits.
Of course, in a novel, I have to figure them out. How many spells can they fling about until they are tapped out? How long can they trudge before they need rest? (Though it's amazingly easy to have them walk farther if you need to use up more time.) At what point will they give up going on even though they have to find the villainess before she does something they know will be disastrous, even if they know no details? Especially when one is burning with the desire for revenge against her?
Fitting the heavily predetermined spells of the RPG (however "house-ruled") does impose limits. It also means keeping count of how spells the characters cast. (Eh, and maybe throwing in a magic object or two to give them powers that they need.)