Idiom of the day :- bite off more than (you) can chew / (To) bite off more than (you) can chew
Usage, examples and definition of the Idiom :-
To try to do something that is too hard to do for you to do, finish, etc. ex. “He started to run the marathon, but after about 10 miles he realized that he had bitten off a bit more than he could chew.”