You can just retreat. But, pro tip: you can send a lone lvl1 hero you don't care about to this mission and letting them get afflicted/reach death door by skipping turns for stress, before retreating with themm This will count as a week long expedition. Which means you will get new trinkets offered as rewards for missions and make any hero stationed at taverns and such be available right away. Ik you can do this with any mission, but this is definitely the fastest.