It will be impossible to commute from to Cedars or UCLA from Simi Valley or Santa Clarita. The distance is not a problem. However, traffic on Interstate 405 tends to be horrendous and moves at a pace of 25 miles per hour.
You will want to live in places such as West Hollywood, Santa Monica, Brentwood, Westwood, or West Los Angeles for the shortest commute times.
When people refer to the Valley, they mean the San Fernando Valley, which is home to communities such as Canoga Park, Reseda, Winnetka, Sherman Oaks, Tarzana, Woodland Hills, Encino, Van Nuys, Granada Hills, Panorama City, North Hills, Northridge, etc. Most of these places are located off Interstate 405 and will involve somewhat of a commute, but not as lengthy a commute as Simi Valley or Santa Clarita.