The student pilot was preparing to do the engine run-up of the light-sport airplane by increasing the engine rpm's to 4,000 rpm's. He reported that his right foot slipped off the brake pedal and his foot partially came out of his shoe. While attempting to reset his foot in his shoe, he released the brake. The airplane turned and went off the paved surface and went into a cornfield. The front of the airplane sustained substantial damage when the front wheel/gear struck the field.