Change the Killing Floor server to Long!
At the moment it is on Short length and Normal difficulty, while bumping it up to Hard may dissuade too many people, short has always been.. well a bit too short.
All I can say is that we've always played KF on the Long game, no matter the difficulty. Excepting those few times we've jumped on the PCG server.
Once we got our perks up to speed we hit Hard and Long and have never looked back.
(...and that's what she said! / ...and in the game!)