The first five miles are the most dangerous - many riders of varying experience packed tightly together on city streets with downhills and tight corners. You have to really watch yourself.

Otherwise, as the other posters mentioned, you'll just need to pay attention, especially when passing.

The ride is well supported. It's a fun ride to do with friends. The course is not particularly scenic.

Weather - It's the Pacific Northwest, it could be sunny and 90 one day and 50 and cloudy the next. You'll just need to check the forecast in the days leading up to the ride. Typically you'll get a tailwind (but no guarantees).

Cannot help with Mountain Bike rentals as I live in the Seattle area. You might try posting in the Oregon forum over on mtbr.com