If I recall correctly you can catch the Astoria bus from the airport. The OR coast is perhaps the most scenic part of the TA so if you have time I'd start in Astoria and you might consider riding down to Florence instead of cutting across to the Corvallis area. When the weather is nice you generally get great tail winds out of the north along the coast so maybe another reason to continue down to Florence.The ride from Astoria to Florence is indeed very nice, I wouldn't categorize it as the most scenic of the trip though.
When I did the TA we started in Newport so we could ride a bit of the coast. Since there were three of us and the bus had rack space for only two bikes we rented an SUV to get there from Portland. We flew in late rode to a motel, rented a car and did a day of sightseeing by car in the Portland area before driving to a Yurt in Beverly Beach State park, and then dropped the car off in Newport in the morning where we started our ride.
If I was to go again and if I wasn't in a hurry I might do as bbarrettx suggested or I might follow the ACA Astoria route.