If you can only handle one outdoor run per week, try to make it your long run, so your body gets used to road running for long distances. Depending on how strict your gym is, you may not even be allowed to stay on a treadmill for more than 60 minutes. Running outside for long runs also means that you won't have to deal with getting bored doing double-digit miles on the treadmill.
And speaking of boredom, you can try an interval workout on the treadmill to mix up your routine. They'll also help get you marathon-ready, too!
In some respects, your training treadmill may better prepare you for race conditions than outdoor running because your marathon will most likely be in warmer weather. Running at room temperature will help you get acclimated to race-like conditions.
Also see: 4 Boredom-Busting Treadmill Workouts
7 Commonly-Asked Questions About Treadmill Running