Although i do not want her to be with the duke, you are right to say that it'd be unbelievably stupid of her to leave, since he'd find her easily and that would cause even more tragedy. The duke would surely kill anyone who would aid her. If she ever plans to escape, she should fo so after at least a year of proper planning, to ensure she won't get caught. And even in that case it'd be unbelievably risky if you consider how cunning the duke is.
Am I the only one who doesnt want her to run away?
She doesnt strike me as achildish person who would leave like that on a whim. She is a honorable lady who is stright forward with her feelings. And quite honestly, she isnt stupid enough to think he wouldnt find her right away.
Get it together people! This isnt a shojo manga!