If you are biking to Astoria, I do recommend taking the 202/47/Banks-Vernonia trail from Portland to the coast. It is very rural and very quiet in terms of traffic and people. It drops you off in Astoria, the northernmost point on US 101 on the Oregon Coast.
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There aren't many services, but there's just enough. Grocery stores can be found at Banks (fairly sizeable) and Vernonia (less so.) Only a couple of country markets with short hours in between after that.
The route from Astoria to Hillsboro (Portland westside suburb) is 100 miles. You can break it into two days by camping at Big Eddy County Park which is about 60 miles in. Few, if any, lodging options are found on this route so if you are doing a credit card tour you should be prepared to do the 100 miles in a straight shot. Possible, but there are two small passes to contend with on the route.
If you want to take a bus, The NW Point bus goes to Seaside/Astora, and runs twice daily.