The Portland Marathon is well organized and a lot of fun. The course itself is relatively flat, with the only uphill being a gentle rise to the St. Johns Bridge. The rest is flat, until you reach the gentle downhill slope leading into downtown portland.
The year I ran it, I tried to follow the recommended training goals but gave up pretty quickly and just ran as much as I could. 2 weeks before the race I ran my one and only 20-mile training run. I felt fine the entire race and was actually surprised by how much energy I had to finish. I kept hearing horror stories about people shutting down at the 20-mile mark. In the end I wished that I had pushed it a bit harder. Pay attention to your body.