That's actually quite a journey by foot. The Bay Bridge was not built until 1936. The San Mateo bridge not until 1929. From SF to Oakland around the bay is in the range of 80 miles. On foot at an optimistic 10 miles per day, that's 8 days of travel.

One thing I always try to consider when going to see friends and family is how long would it take me to walk there. In most cases, it would be days if not weeks. Evacuation on foot is not something most consider, especially if bridge infrastructure is compromised. Add in a child or two or an elderly relative/friend and you're a long way from nowhere on foot.