Esta has more healing magic, yes.  He'll wait until he gets the [fatigued] condition, and then use it on himself.  If Merrin thinks she can still outlast him under that premise, she's welcome to try.

Twelve miles would be trivial with a road or an Air Walk.  Can they do it on rough terrain?  That sounds like a more interesting challenge.

Esta has more healing magic, yes.  He'll wait until he gets the [fatigued] condition, and then use it on himself.  If Merrin thinks she can still outlast him under that premise, she's welcome to try.

Twelve miles would be trivial with a road or an Extended Air Walk.  Can they do it on rough terrain?  That sounds like a more interesting challenge.