The distance to the beach is written on the trail surface every half mile (mostly), along with a painted line. If you look for the markers, you'll spot them. Some of them have been partially or completely paved over, but there are still quite a few that are visible.
PS, my wife has ridden a bike about 10 times in the last 20 years and she's did a 35 mile round trip on the SART to the beach and back (I think it was her 7th or 8th ride) with me. Generally coming back is much easier than going down there because of the ocean breeze, so even if you're tired making it back home is usually easy. Santa Ana winds may nullify all of that of course.
Thanks! And, if I may say so, 2010 Roubaix Expert? You have EXCELLENT taste in bicycles, my friend
Just got my white/carbon from The Path a couple weeks ago and love it.