Can you link you approximate route?
In general, the Bay Area is one of the most bike friendly areas. That said, there are a few bad sections of freeway where it can get sketchy in cars. Those are just neighborhoods to avoid.
To answer above questions: bart is actually BART (Bay Area Rapid Transit), its ~$2 each way for a short distance. Since it'd cut 5 mi off an 8 mi commute hardly worth biking at that point. Especially if the BART station is not adjacent to either the start or end point.