Would this be okay marinating for many hours?

thank you so much

You always have to be careful when marinating meat that the marinade doesn't ruin the texture of the meat. Extra long marinating time is not a great idea.

Are you using an unpasteurized miso? If so, this will be a fairly potent enzymatic marinade, and you'll have to watch it carefully so that it does not turn the outside of your steak to mush. If not, it will be much more forgiving.