Dont let it keep happening, check the thermostat and/or water pump very soon. I had a shattered piston caused by corroded water pump fins. The fins were completely corroded so no coolant could circulate, causing overheating, which expands the metal in the block, leaving no room for the piston, causing it to shatter. Same symptoms as yours. It wouldnt overheat until about an hour or some hard abuse. Check out RockAuto.com, I picked up a water pump for like $16 or something like that. Thermostats are a super cheap fix too, dont wait too long or youll be looking at a $2000 bill for a rebuild.