I gotta say, while a ton of the simple people who are so disconnected from reality due to bordeline sociopathic MC's being common in isekai/manhwa, it's actually really nice seeing Nao have PTSD (though the mangaka could stand to take breaks from Ichijyou being edgy and going on training montages to focus on her character more)
if she is having so much trouble walking there why not just have MC give her a piggyback ride? he picked up and tossed a tanker truck with no issue back in ch9 (and that was even before he killed the minotaur)
Art isn't showing it, story isn't showing it, this is just a hunch, but I think that nao's thank you and I'll try my best aren't from genuine happiness, but fear of the MC, also MC hopefully let the boss kill him before for the sake of stats