A hill shows you the distance your eyes had been calling small.

Estimating from a distance is a different activity from doing the thing. A slope looks like a short walk from the bottom, and the legs report something else halfway up, because the eyes never had the information. Most underestimates work like this: they are made by a part of you that will not be doing the work.

Where you have judged something easy, find out whether you judged it from the bottom of the hill. Try a small portion of it before committing to the schedule. And when someone who has done it tells you it is longer than it looks, that is data rather than discouragement.