Do you want to limit yourself to paved trails like the SART, or are you OK riding on roads? There are tons of routes in OC, from the southern and eastern end of the county with as much climbing as you'd like, to the flats of the central county out to the beaches. Check out the websites of Bicycle Club of Irvine and OC Wheelmen for example (clubs I ride with). Some my recent rides originating in Lake Forest or Irvine:
Newport Coast, Pelican Hill, Vista Ridge, Turtle Rock, Santiago and Silverado Canyons, Cowan Heights - 85 miles and 5500 feet of climbing:
Downtown Long Beach and return - Main, Sunflower, SART, PCH, Beach trail in Long Beach, 80 miles and just 1,010 feet of climbing.
Irvine Center/Edinger, Red Hill, Santa Clara, SART, PCH, Bayside, Irvine Terrace, San Miguel, Bonita Canyon & Harvard. 42 miles with 900 feet of climbing.
Trabuco, El Toro Road, Santiago Canyon, Jamboree, Portola, Irvine Blvd - 34 miles, 2400 feet.
Hard to get anywhere completely avoiding roads. You can ride all the way from PCH/Bayside out to Irvine Center Drive & Research using the San Diego Creek Trail. You can also exit at Harvard then pick up Peters Canyon Trail to Hicks Canyon and ride it all the way to Portola Parkway near Jeffrey. There is the Aliso Creek trail that runs from Cooks Corner down to Laguna Niguel Regional Park. I haven't tried to ride the trail in a long time so not sure if it is completely offroad.