If you're in a hurry, take 101 to Salinas and CA 68 to Monterey. Monterey to Paso Robles via this route is 2 hrs each way. I'm not sure how much further to Avila Beach -- another 45 minutes or so? So call it almost 3 hrs each way the FAST (but boring) way.
If you take coastal Hwy 1 in one direction, plan on spending most of the day doing that.
I have taken my Mom & aunt from Monterey, down Hwy 1, to Hearst Castle at San Simeon, taken the 1 hr tour, had dinner, and then scooted back to Monterey via 101, all in one day. But it was a *long* day. And all we did, other than taking our time seeing the sights on Hwy 1, was do a 1 hr castle tour and eat dinner.
I agree with the others. If you actually want to see anything, stay overnight in Monterey.