Going to put my 2 penneth in.
The courses I have helped set out for the various Winter Leagues over the years have tended to have an average of around 38yds. Weather and angles play a good part in it.
Our club tends to start planning any course months in advance by looking at moving a firing line forward or even backwards looking for unusual target safe target positions, foilage that could cause misjudging of ranges and finally the ground itself with any trees and dips. Also balancing out the difficulty for left and right handers so if there is a lane that is uncomfortable for a lefty there will be one for a right handed shooter as well.
I find the best courses are not those that are 'long' but those that you need to think a little before pulling the trigger. Subtle shifts in the targets on the lanes mean that you have to constantly think about wha the wind may do.
For a GP course I think an average ditance over the whole course of around the 43-45yd mark is perfect if you have had to think about each lane. I personally, get very despondant when a course seems to have a majority of the targets over 45yds and welcome the introduction of the 15mm kills.