86 San Luis Rey bike path out & back. 22 miles, 281 ft el, 13 ft/mi

0.0 N Coast Hwy 101
 0.7 L Eaton
 0.8 R Broadway
 1.2 L Cassidy
 1.3 R Pacific
 3.4 R Surfrider
 3.5 L Cleveland
 3.7 L Park
 3.7 R bike path
11.0 U College-path end
18.3 = exit on Park
18.3 R Cleveland
18.5 R Surfrider
18.6 L Pacific
20.5 L Cassidy
20.6 R Broadway
21.1 L Eaton
21.2 R Coast Hwy 101
22.0 return to start

As flat a route as you will find anywhere in the county.