Well, you have a good start at becoming a better rider, just by being on this site. You'll get a lot of information here.
Do you know of a cycling club in your area? You should look into that. The best way to get better is to ride with more experienced riders and learn from them. Plus it motivates you to get out and ride if you have a plan to meet at a specific time. Ask around at your local bike shops for any info on a club, or search online.
If there aren't any clubs in your area, if you could at least find a friend or two to ride with, that would be great.
Seattle to Portland is a great goal. My friend did it a year ago. The way you train to do any length mileage is to work up to it. Find a century or two to do before Seattle to Portland. Work up to those by doing one long ride per week, probably on the weekend. Start with 40, then add five or ten miles a week until you can do at least 70 or so a week or two before your first century. If you find you are having a lot of trouble accomplishing a certain distance, like 60 and can't imagine going further, then stick with the 60 or even 50 for a while so your body can get used to that ... then move on. During the week, try to get in two or three shorter rides. All of this is developing base miles, which will get your body used to riding and ready for S to P.
When S to P is maybe two or three weeks away, you might want to do a back to back long ride of 70 to 75 each day, to simulate your goal. You don't have to do the full distance in training in order to do it at the event. you might want to do shorter back to back rides even and build up to the 70. But first find a century and train for that.
Good luck!