Having driven that route many times I can confidently assert that the Seattle → Eugene leg will be much closer to 5 ½ hours as the traffic between Seattle and Olympia can be absolutely brutal. Even on a good day expect 5+ hours to make that drive. For reference, the real-world drive time between Roseburg and Spanaway (10 miles south of Tacoma) is 6 hours on an average day without any traffic jams. If you leave the freeway at any time the drive time will increase significantly.