bite off too much

“Don’t bite off too much,” Browne said.

And, she adds, don’t bite off too much at once.

“We all bite off too much sooner or later. But some students don’t know when to spit their mouthful. They burn out. They quit, or botch their exams. Some crack.”

Don't try to bite off too much, don't get too aggressive.

Don’t bite off too much, too early.