I'm not sure why he thought of it, but a trek is a long, difficult or adventurous journey. It took on that meaning during the Boer migration into South Africa (it was originally a Dutch word meaning to draw or pull, as in to pull a wagon), and so I'd imagine it came to be associated with the mythology of the frontier, with grand pioneering quests and arduous adventures. Roddenberry gained a lot of his prior TV experience writing Westerns -- which were a ubiquitous part of TV culture at the time -- so the term "trek" and its frontier associations were probably somewhat prominent in his awareness. So I guess when he decided to do a show about pioneers journeying into the frontiers of space -- especially one that he pitched as "Wagon Train
to the stars," in reference to a famous, successful Western series whose format he wanted to emulate -- Star Trek
made sense to him as a title.