Walking distances may seem more un-doable than they really are. It can help to just start and eventually you'll get there (within limits and if there are no time constraints). With swimming distances it's the opposite I've found. Usually you look across a body of water and think it's a short swim, while it's anything but that.

A long time ago I decided to swim to the other side of a lake, thinking it was doable, only to find out that I was not going to make it. I turned back, but saw that I was over halfway and back was even longer. So I continued instead and definitely was lying on the shore for a while like they do on tv and films, after some shipwreck scene. I remember resting there for a long time, and trying to get to terms with the fact that I had to swim the same distance back.

It's a long time ago, but I still remember the lesson: do not underestimate distances for swimming.

(A similar lesson on the same lake: when canoeing start against the wind, so you know you can make it back. Starting out paddling with storm force in the back is very relaxing, less so when trying to go home your progress is only in centimeters)