Like in improvisational style the players are often making decisions on things which do not have answers other than the ones GM is making up on the spot, and the players' answer is not something that can fully (or even meaningfully) inform the GM's decisions.
This is what mechanics are for.
And also just talking to one another.
In my Prince Valiant game, when the PCs are travelling from A to B we all look at the map together, be that the map of Britain on the inside cover of my Pendragon book, or a map of Cyprus that we Googled up, or whatever. And when I narrate that
you come to a forest or
as you pass through some badlands or whatever, we'll look at the map and work out where those events are happening.
This is close to the opposite of map-and-key play: there is no secret map behind the GM's screen. There is no tracking of miles-per-day travelled and rations and the like. There just the loose narration of time passed before the interesting event is introduced.
Suppose time actually mattered: eg suppose that one of the PCs had to get from A to B before an enemy army arrives at B to lay siege. Then a difficulty would be set - maybe an opposed check, maybe versus a fixed difficulty extrapolated from the fictional situation - and resolved via rolling for Brawn plus any appropriate skill (eg Riding if racing on horseback, Agility if running, etc).
In this sort of game the GM is quite unlikely just to
decide that the PC can't get from A to B in time. That's not the sort of improvisation that "no myth" RPGing relies upon.