I'm new to 4x5 and am in the process of acquiring gear. Camera, enlarger, film, paper, etc..

I found a camera on ebay, a Cambo SC in nice condition, and mounted a Schneider Symmar S 135mm on it. I don't have all my gear yet, but am fiddling around with the camera mounted on the tripod and pointing it out the window at some trees (it's COLD outside here in Michigan today).
I put a black t-shirt on the camera for a dark cloth and attempted to focus an image on ground glass. Aperture's wide open at 5.6, and the ground glass is being illuminated, BUT...I can't get the image in focus. I get close to being in focus, but end up with the front and rear standards so close together on the rail that I run out of space between the standards before I get objects 50 feet away to come into focus.

What really basic thing am I missing here? My photo background is in medium format (retired pro protrait photographer) and 35mm.