When you're thrown into a casket, locked in, and set on fire oddly enough you end up with quite a bit of time to think about things before it gets really uncomfortable. It isn't the flames that end up doing the damage, but rather the heat.
If it had been a nice, pleasant pine box I would've been good. I could've punched through that sucker like tissue paper. It would've taken forever to burn. Think of how long it takes for logs on a fire to burn all the way through. Your average casket's got a good inch or so of wood between you and the outside world, and wood is a shitty conductor. Like Southeastern levels of shitty conductors.