Sounds like you better get it towed to the shop for a new heater core ASAP. Otherwise it might rupture and spew scalding coolant all over your lower legs and that will not make for a fun evening. Technically, turning the heater to the coolest setting should keep most coolant from circualting through the core, but do you want to risk it? Please for your own safety do not chance it. Spend the cash and have it done.