If you want to keep it simple and local to Santa Monica: one idea is to start at the Strand (bike/running path along the beach) and head toward Will Rogers State Park. Cross the PCH there and run up Temescal (decent sidewalk) and go up to Temescal Gateway Park (trails, etc...) and turn back.
Here's something I whipped up quick:
https://ridewithgps.com/routes/41850...2a8xwNjIZVuKE6
There's also Griffith and Runyon if you want something more trail-oriented.