Firstly, I should mention that I haven't seen the entire trail
We hired a bikes for a day and rode part of it. We are slightly to moderately fit. It was an easy day ride from SF city across the bridge to Sausalito and on to Tuburon with a ferry back to SF.
Cost about $90 each for bike hire and ferry ticket. Lots of little shops along the way in Sausalito and Tiburon to buy snacks or stop for a meal/drink.
SF weather is The BEST for bike riding, even in mid-summer. We will do it again in mid-winter because we loved it so much.
Great views back to the bridge and city.