The only decent mileage runs these days from the Bay Area are on Alaska Airlines, which still accrues miles based on distance when on Alaska metal, and, to a limited sense, some of their partners (Qantas accrues 100% miles plus any elite bonus). You could fly to the east coast or Hawaii 15 times, accrue 75,000 elite qualifying miles, and be a top tier flyer there. Alaska does have some east coast service from San Jose including daytime nonstops SJC-EWR and SJC-JFK, but only the daytime JFK flight would allow a same-day return. SFO offers a lot more options and nonstop destinations, and OAK offers some Hawaii flights in addition to west coast flights. Occasionally you might be able to make a connection in SEA, LAX or SAN work for extra miles, before continuing onto the east coast. Just depends what the fare allows and when you depart.
If you intend to stay a United flyer, an entry level flyer there would accrue 5 elite qualifying miles per dollar spent, so a $300 east coast ticket would bring in about 1,250 miles (compared to 5,000 at Alaska). With Alaska top tier elite status you could bring in over 11,000 redeemable miles. You don't see a lot of AA or UA mileage runners these days unless it's end of year and they're short some miles to requalify. AA had a bunch of $100 range round trips to the east coast this week (PIT, BDL come to mind) but they'd only bring in 250 miles (5 miles per dollar spent pre-tax which would be around a $50 base fare).
I've done AA mileage runs over the years with a few folks from Northern CA but now I pretty much do at least a one night hotel stay or a weekend. You might post on the Mileage Run Discussion forum once you decide what airline you'll be flying with, and you might find some interest there among other Bay Area flyers.
https://www.flyertalk.com//forum/mil...iscussion-627/