Doing any kind of lap route in Inland SoCal is gonna be 35ft/mi or more. That's just how it is. I live at ~1,300ft, and two subdivsions over (about 3 miles) it's 1,800ft. I can climb 3,000ft simply by riding 15 miles east.

Sooo... six laps around the old AFB now non-operational airport, bolstered by about twenty half-mile laps around Parking Lot C, just in front of the airport terminal. This is SoCal. 20ft/mi is as flat as it gets unless you go to the beach.

Even just riding beside the Santa Ana River for 20 miles picks up about 10ft/mi when going "downhill," and another 700ft or so on the return trip.

If it were one-way, I've found a route that can take me 70 miles from my house to the Pacific Ocean with barely 800ft of climbing. But the return trip is like 2,500ft.

I ride around that old AFB a lot. Closest thing we have to flat. And at that, the east end of the runway is 70ft higher than the west end.