Start riding the trail at the northwest end at Sir Francis Drake and Platform Bridge Road. You can park in some of the dirt turnouts on Platform Bridge Road and then cross the old bridge over Papermill Creek to get on the trail:
Most of the trail through Samuel P Taylor is paved from that point until you get in the area of the park entrance (by the day-use area). The trail is very level, since it is on an old railroad right-of-way, and passes through deciduous forest which changes to redwood forest:
A little way past the day-use area the pavement ends and the trail becomes rock/gravel/dirt. Easy to ride with a mountain bike, cross bike, and most hybrids. Skinny tire (narrower than 28C) road bikes may be a bit uncomfortable, but it is still passable. After about 100 yards or less the bike trail crosses over Sir Francis Drake via a bridge and continues on the other side of the creek from the roadway:
The trail continues as rock/gravel/dirt for about the same length the paved trail section was, around a couple of miles, until you get to another bridge that crosses back over the Lagunitas Creek to the other side and ends at the roadway of Sir Francis Drake. This bridge goes by several names, but is mostly known as Inkwells Bridge (as the popular Inkwells swimming holes are right beneath the bridge):
If your beginner is only able to ride on mostly level surfaces then this would be your turn-around point for this out-and-back ride. There is no more bikeway on the other side of Inkwells Bridge, instead cyclists wanting to ride further upstream toward Lagunitas, San Geronimo, and Fairfax must do so on Sir Francis Drake Blvd. This roadway can be quite busy and scary for a beginner in a lot of places (especially in the redwood grove Sir Francis Drake passes through going eastbound between Inkwells Bridge and Lagunitas Grocery and Deli) even though there is a bike lane the cars get pretty close and speed.
If your beginner can handle a little bit of a climb on a dirt fire-road, you can ride up to view Kent Lake and Peters Dam. If you want to do this, at the end of Inkwells Bridge go to the right for a few yards and carefully cross Sir Francis Drake at the first gated road entrance on the left that you get to. This is Peters Dam Road. You do not want to cross the roadway bridge and use the second left entrance (where there is a parking lot, may or may not be in use), which is Shafter Grade Road.
Peters Dam Road will follow along the west side of Lagunitas Creek and climb up to the level of the dam. It is the easier route to go as opposed to Shafter Grade Road on the east side of the creek. Once you are done looking at the lake and dam you can reverse direction and return down to the Cross Main Trail (there are no trails going around Kent Lake other than fire roads that climb up above it, but that would be beyond where you want to take a beginner).
Climbing Peters Dam Road.
Looking back down Peters Dam Road from the dam.
Looking across the dam, which you can ride on.
Kent Lake
Looking down at Lagunitas Creek from the top center of the dam
Do a Google map check of all this area to see what you think. It is a fun trip that your beginner will love.