Here's the official shuttle bus description:
http://www.united.com/page/article/0,,1121,00.html
Free bus transportation is available for United and Continental customers with connecting flights. United pick-up is at gate 92, and Continental pick-up is at gate 32. Bus service is available every 20 minutes between 6 a.m. and 10 p.m., seven days a week.
Note the frequency. I have exactly the same issue catching BART, arriving in SFO, except there's usually a next train.
Print a map in advance, so you can swing by gate 92 and make a quick call. Be ready to walk as a backup, unless you have enough time to chance the bus.
Rock and a hard place, I know. Good luck.