Consider
電車がないなら、歩くまでだ。
This seems to literally mean "if there is no train, it is until I walk", which is hard to parse. What is going on with "it is until I walk"? I assume the sentence idiomatically translates to "if there is no train, I will walk". But if that's the case, why not just say:
電車がないなら、歩く
Is the まで in the original sentence hinting at something like "I will walk until I get to my destination"?
It’s like saying the action of 歩く is the only thing you have to do under the given circumstances. There is nothing more for you to do. You are determined to take the only option left for you: walk.
まで marks a limit as in other usages of the word.
Just to add on to aguijonazo’s answer, here is the relevant definition from 明鏡:
❺《「…までだ」などの形で》物事を限定して、他に及ばない意を表す。
「電車がないなら歩くまでだ」
「失敗したらそれまでだ」
「その勝負、もはやこれまで」
It’s similar to 歩くしかない.
