Maybe some kind of corn snake? He's in southern California, and the bike path was along the San Diego river.
Since he's a gopher snek, he's probably someone's PA, off to fetch a half-caf oat milk latte. And he's harmless. Also, he's native to California. Leave him be.
https://www.calacademy.org/blogs/naturalist-notebook/ask-a-naturalist-rattlesnake-roundup