In late June Sunriver can be extremely hot. Last year I ran about 20 seconds slower per mile on this course, largely due to the heat. Although, I am sure that the altitude didn't help either. The course is beautiful, but the outside portion of the loop (which you do twice) is really exposed to the sunshine. The aid stations are plentiful, and the swag is ok, but there is a lot going on and some of the marathons thunder is stolen by the multitude of other events.
If you are a sub 3:30 try to get near the front of the pack. The start funnels quickly and you can easily get jammed up for the first 1/2 mile or more if you are not careful.