Frequently when I speak with people from off-island, they are surprised to hear that they can visit Whidbey Island without taking a ferry. It’s true, there is a bridge that can bring you to Whidbey Island from the mainland! Those of us who live at the north end of Whidbey Island rarely take the ferry over to the mainland, even when we are going to Seattle. The trip over Deception Pass Bridge, then to Seattle via State Route 20 and Interstate 5, is usually faster (and more predictable) than driving the length of Whidbey Island and taking the ferry from Clinton to Mukilteo and the mainland. From Oak Harbor it is about 30 minutes via Deception Pass Bridge to I-5 at Burlington(north of Mount Vernon), and shopping in Bellingham is less than 1 hour away. From Coupeville it is less than 45 minutes via the bridge to the I-5 at Burlington.
Once you get south of Greenbank, the travel equation begins to weigh in favor of taking the ferry to the mainland. But weather and traffic (especially weekends during the summer) can make the wait for the ferry unpredictable, so many people will still travel via the Deception Pass Bridge, or else leave adequate time to account for possible delays.
So in summary, to get to Whidbey Island without taking a ferry, take I-5 to Burlington, then SR20 towards Anacortes and Whidbey Island. This route takes you over Deception Pass Bridge and onto Whidbey Island from the north. From Sea-Tac airport it is exactly 2 hours to Oak Harbor under normal conditions.