I think she didn't leave town sooner because she was pretty much focused on being with Tony (plus, of course, her "back-up men" who she kept in touch with just in case things didn't work out with Tony). But as others have mentioned, by this point she's pretty close to the end of the hallway. Amy's about to figure out that she's been robbed, she's been cut off from other sources of funding like her grandfather's bank account, and I can see her figuring this is the time to cut and run - probably to another man. Once in California she could find a new circle of friends who wouldn't even know she'd had a baby. She could tell Mark the baby was with the grandparents, she could tell Cindy and George the baby was with her and Mark.
Tink
You hit the nail on the head. I think that's exactly what she had planned, but CA showed up. If CA had given her another day she would have been long gone by any means possible, probably ask MH to pay her way there and never contact GA/CA again...