The non-stop DCA-SFO flight often sells out F in advance, making it difficult for everyone. There are only two DCA-SFO nonstop flights (one by VX and one by UA), and premium cabins on these flights fill up quickly due to flyers' preference for DCA and nonstop flights. The flight is often "elite heavy", so any remaining seats will likely be taken by the time the 6 hour mark rolls around. Your odds aren't particularly good, I'm afraid.

If upgrading to F is important to you, you can try to do a same-day confirmed change for $50 and switch to a connecting itinerary, DCA-DAL-SFO. If this floats your boat, upgrades are much easier to snag on these flights. In addition to having a greater likelihood of success, it'd also cost less to upgrade both shorter segments compared to the one long DCA-SFO flight.

Here are the prices:

DCA-SFO (nonstop)
$399 MC-->F
$169 MC-->MCS
$240 MCS-->F

DCA-DAL and DAL-SFO (each leg)
$139 MC-->F
$69 MC-->MCS
$70 MCS-->F