It is not so hard. At worst, you can just fall in behind a rental car shuttle bus as it travels the terminal loop and follow it back to the rental car center (although you'll stop at the T Blue Line station unnecessarily on the way).
Allow a bunch of time, though. About 30 minutes to fight your way back to the central rental car facility, drop the car off, and ride the shuttle back to the terminal. And that's not counting stop-and-fill-the-tank time if you don't get that done earlier.