California, Santa Ana River Trail near the beach areas.
Santa Ana River Trail alone is 60 miles round. But there is also the San Gabriel River Trail 10 miles away which is easily connected by riding on Pacific Coast Highway. The San Gabriel is about 65 miles roundtrip.
You can connect them and do an out and back and get an easy 100 miles.
Much of the trail looks like this. Nice, smooth and pretty flat.
A nice 50 miler with one of my buddies on SART.
