From BART to UA check-in is 8 minutes max. Once you step off BART go upstairs to the "Air Train" station. Get on the 3 car tram, get off at the 2nd stop for UA. Go down the escalator and into the terminal. Its the easiest ride I've had to SFO. Now if you want to walk from the BART station, that would be a hike, especially with bags.