You could start the trip anywhere in the loop. We drove to Willows and parked in the Walmart lot (with permission from them to leave the car in their lot for two weeks).
Greyhound has direct service from San Francisco to Arcata, with stops at Oakland, Santa Rosa, Laytonville and Eureka.
Weed has Greyhound service.
Chico has Amtrak, Greyhound and commercial air service.