The walk from the main terminal to the Marriott is not all that easy. No sidewalks. One must either walk on the roads circling the parking lot, or cut across the parking lot, then across the same circling road, and then across private parking lots for office buildings separating the terminal from the Marriott. If I had not been there before, I would not strike out on foot after midnight on foot, unless young enough and no luggage. No threat of petty crime per se, just a dangerous kind of walking environment.

My suggestion would be to decide whether you want to book a room there and then use the shuttle bus, or if you really prefer to stay inside the terminal. There are some fairly comfortable rows of seats downstairs near the international arrivals and customs facility (far western end) where I might sit and even snooze for a while. Also smaller number of seats near the main exit point for domestic passengers on incoming flights, which is where you will probably exit into the baggage claim area from your incoming flight.