The area of around Tustin/Villa Park/City of Orange in Orange County. All are near the NW corner of the Santa Ana Mountains so you'll ahve access to Santiago Canyon Road, Jamboree (above Irvine), and few fire roads if you do some MTBing. For flat interval work you could always use the Santa Ana River path; its crowded mid-day on the weekends, but pretty vacant on weekend mornings (before 9AM). I would NOT recommed South Orange County (from Irvine southward, south of Hwy 55 on either I5 or I405). Traffic between there and Long Beach is a nightmare both in the morning and evening; from the Tustin area to Long Beach is crowded, but a lot easier than the South OC traffic bottlenecks.

What part of Long Beach is your employer going to move to? That could influence whether to look in Pasadena/LA County or Oorange County.

And maybe someone could chime in on the South Bay area? Torrance/Palo Verde/Hermosa Beach/Redondo Beach?