Suppose ATC says "Big Jet, cleared to cross runway 36, expedite" and you acknowledge "Cross runway 36, wilco, Big Jet". As you start to move forward you immediately realize that you are unable to go as quickly as you expected (perhaps there's an incline or a gust of wind -- I'm not sure what's plausible). At the point you realize this, you can still hold short the runway. Are you obliged to stop and reply 'unable' or is that ill-advised?
If you feel you cannot safely comply with any ATC instruction, you can simply reply “Unable” and the controllers will amend your clearance. Remember: you are pilot in command; you are the final authority on the operation of that aircraft.
So the scenario might play out as:
ATC: “Bigjet, cleared to cross Runway 36. Expedite crossing and report clear.”
Bigjet: “Cross runway 36 and report clear, Bigjet.”
Bigjet: “So-And-So Ground, Bigjet, Unable. I’ve got a problem here.”
ATC: “Bigjet roger hold short of Runway 36 for arriving traffic. Will you be able to taxi after traffic clears the active?”
Per Part 91, You are cleared to cross until/unless another clearance is obtained. https://www.law.cornell.edu/cfr/text/14/91.123
If you don't think you can safely comply with an instruction, don't comply with it. If you can remain short of the "hold short" line, stop there and inform ATC. If you have already crossed the line call tower and say "crossed hold short, unable to expedite" or something to that effect so they can have landing traffic go around.
Its been said quite a few times, the pilot is in control of the plane, not ATC. You can agree to something and then later realize you can't comply, that is fine, as long as you communicate with ATC.