I'm going to Alaska via cruise and I signed up for a 4-6 mile hike?

Now I'm 27 years old, not overweight but I'm not like a daily exerciser or anything. Is this going to be too much for me? I'm thin and I can walk a couple of miles easily but I'm wondering if I got in over my head signing up for a 4-6 mile hike up a glacier. Is that a practical activity for me or should I cancel it and do something else. I just thought it would be cool to do that in Alaska; but there are other less strenuous activities. Thanks!