We live in Nanaimo and usually make it in two full days of driving. It works well if one of you is a morning person and the other is a night person. We usually make it to either Grants Pass or Medford the first night. The next day I wake everyone up around 6 and we start moving again and just boot down the 1-5. We usally split it so I drove until around 2 and then Dh takes over until around 9. Without kids I don't see any reason why you couldn't do the marathon drive one of the ways.
But if you are planning to visit San Francisco it's going to add a day to one of the drives because it is quite a ways off the 1-5 so you can either take the detour or go along the 101 one of the ways. Personally I'd go with the 101 and take the time either going down or coming back. It's really pretty.
Your biggest variable in time is going to be the border. Since you are on the Island the sooner you get there the better. I personally am a fan of the 5am sailings but if you can get over the night before and cross at night it's really fast and you can make it through a lot of Washington without traffic which is also nice. What we did that worked out really well one year was the first night we drove to Chehalis, then the next morning we moved over to the 101 and took the coastal route. We spent another night in Southern Oregon, then another in San Fran, then the next day made it to
Disneyland. So it was 3.5 days along the coastal route with a half day spent in the piers at San Francisco.
Also some tips we've learned over the years. Don't speed in Oregon, after coming out of Cali where everyone is going 85 m/hr Oregon seems really slow but don't speeed there, we got a $500 speeding ticket within five minutes of crossing the border a few years ago...it got reduces to $200 but still it was a bit shocking.
Motel 6s are super cheap but the beds are tiny, they are kinda cruddy, and have no clocks. I personally would rather budget around $100 and get a nice hotel even when you are just stopping and taking a break from the drive. Nothing is worse than driving tired because of a bad sleep on a tiny little bed! I personally like stopping at the Holiday Inn Expresses because they've got clean nice rooms and a good breakfast bar where they let you take food with you.
And one of my favorite parts of every trip is the start of the second morning. The border between Oregon and California is beautiful, especially at sunrise! There are some nice lakes and mountains.
ETA: Also get your brakes checked before you go. One of the things that drives me nuts about southern california is that the freeways will just stop without a lot of warning. You need perfect brakes for that.