Here it is.

There are three campgrounds along the Smith River. You'll only see two of them from the road you'll be on. Vincent Creek is my favorite. There's a small store/bar 14 miles past Smith River Falls. Once you pass Smith River Falls, there are a couple of places along the river you could camp and then there's no real nice spot to camp until Umpqua Lighthouse State Park. Unless you want to ride 90 miles on your first day, I'd recommend camping at Vincent Creek or Smith River Falls.

If you did want to push all the way to the Umpqua Lighthouse State Park, you might consider William M. Tugman State Park even though it's a bit farther. The hiker biker spots at the Umpqua Lighthouse State park aren't that great IMO. The view at the lighthouse is worth the bypass through Winchester Bay.

EDIT: Follow Shifty and B. Carfree's advice for getting through Eugene - They know Eugene - I really don't