Strategic options when targeted in a papal holy war?
I'm a Lollard king outnumbered 4 or 5 to 1. Should I convert to Catholicism? Will that immediately end the war? I really don't want to be slaughtered and lose land.
Yes. As soon as you convert to a faith that is not "Hostile" to the holy war, the holy war is invalidated and ends in a stalemate (identical to a White Peace, I believe).
Technically you don't have to convert to the aggressor's religion, but that's often easiest, as you get a hefty reduction on conversion cost should you choose to do so.