I probably should consult the instructions that came with the etrex before making this comment, but I don't think it would be able to tell you how far it is to a certain destination unless you had previously plotted it. It does have the capability to mark waypoints etc., but I don't know if you can download maps into it.
In any case, you will probably want to have a paper map (chart) of your proposed journey. Depending entirely on an electronic device that might fail, wouldn't be very prudent. It would be easy enough to check your distances left to go from said map.