erin, the book that HikinJim mentioned has a good reputation. When I started hiking a year ago my principal info source was
http://www.localhikes.com. Mission Trails Regional Park web site
http://www.mtrp.org has simple maps outlining short hikes such as Cowle's/Pyle's and Fortuna. As you work your way east past Lakeside "El Cap" there are miles of trails and fireroads in the Cuyamacas with maps and descriptions at
http://www.cuyamaca.us.
In San Diego we have the choice of coastal, mountain, and desert hikes. Personally, I prefer mountain. You may wish to do all three types. I think that all hikers plan long-day or short-day hikes depending on their schedule from week to week. My short-day hikes include Poway Mt Woodson, Poway Iron Mtn (see
http://www.poway.org), and Rancho Penasquitos Black Mtn (see
http://www.penasquitos.org/map.html). Sometimes I do one San Diego mountain then drive a few miles to do a second mountain (and/or a third).
I get my paper maps at
Map World/Map Centre in Clairemont Mesa. They're small, but the service is good. When I bought my GPS they had the best price.
If you crave elevation above 6,500 feet, you'll be burning gasoline heading north on I-15 to Baldy, San Gorgonio, and San Jacinto. Each of those places has slightly different procedures for obtaining hiking permits, and you'll learn those procedures quickly. I recommend buying an annual parking pass to use in those National Forests. Cuyamaca State Park charges for parking; I have not yet invested in an annual parking pass for the Cuyamacas.