From the coast, the most direct route to Eugene is from Florence on the Oregon Coast via OR 126 (just under 1.5 hours). You didn’t mention where you are traveling from, but it sounds like you may be coming from the north and you could easily spend four days traveling the North and Central coast between Astoria and Florence. Astoria is famous for bed & breakfasts in historical Victorian homes. If you are looking for more ocean beach destinations, I would consider starting in Seaside or Cannon Beach. In Seaside, you may want to try the Gilbert Inn B&B (historic home). In Cannon Beach, consider Song of the Sea B&B (modern home, close to town and beach). Just south of Cannon Beach is the unique French Chateau-like Arch Cape Inn. In the Newport area, you may want to consider the Sylvia Beach Hotel. It’s actually a historic beach hotel, but very B&B like with a literary slant. Rooms are furnished and decorated based on famous authors. Another unique B&B stay would be the Heceta Head Lighthouse B&B, the original lighthouse keepers’ home (near Florence).
Weather is typically mild and rarely below freezing on the coast, so your travel along the coast should not be a problem. You will want to check on pass conditions as you pass through the Coast Range on your way to and from the coast. From the Portland/Longview area, the lowest elevation on the North Oregon Coast is Highway 30 into Astoria (the Washington side of the Columbia is even closer to sea level). Highway 26 is the most direct route to the coast from Portland, but the Coast Range summit is about 1500 feet and is most likely to have mountain travel conditions. The highest point on OR 126 from Florence to Eugene is under 800 feet.
Weather in January is typically cool with highs average in the high 40s and lows in the mid to high 30s. It is one of the wetter months of the year, averaging nine or more inches of rain during the month, so be prepared. Many come to the coast for storm watching this time of year. There can be high winds. The storm tossed seas can be an awesome site.
Hope this helps.