I worked at REI for a number of years. They're all about customer satisfaction. If they say that you can't take it out of the parking lot, just kindly ask to speak to their Supervisor. Explain that you need a longer test ride over a variety of terrain, and they should be ok with it.

Worst case scenario, buy the bike, ride it for however long you'd like, and then return it later. They'll take back ANYTHING. The store I worked at let an angry customer return an Old Navy fleece pullover. Yeah...

Trey