The Whispering Pines Trail race in Tyler, Texas. I looked around for about the easiest 50K trail race I could find and that one came close to fitting the bill. It's 3 - 10.3 mile loops with 960' of elevation gain and loss on each loop for just under 3,000' of elevation gain and loss for the entire 31 miles. Not flat by any means, but not 10,000'+ like some of them. Mostly just rollers, I think the biggest hill is about 200' gain over a mile with several other 100' gains throughout and then lots of little stuff.
Weather is my biggest concern, in Mid-May in East Texas it could easily be 85 degrees and 80% humidity by the time the race is over for me. Humidity is going to be a killer because it has been a very dry winter and spring here and I just don't have the opportunity to train in higher humidity at all. I've kind of got a 5 hour benchmark in my head, but I'm not going to be too upset if I go over that, especially it is a warm humid day.
Good luck on your marathon. Any particular time you shooting for?