Hey!
A local San Diegan! Good stuff. I've just started "hybrid-commuting" myself (I ride folding bikes). Which means that I'll either get dropped off some of the way or all the way to work. The ride home. I haven't done a full commute yet, since I work in labor and its 15-miles one-way! From College area to Pacific Beach. There are some mountainous hills on the way back. I did try it one day on my day off and it took about 4-hours. LOL!
Where exactly or approximately are you riding from-to?
I recently used Sports-Tracker to map out the way back home {
http://www.sports-tracker.com/#/work...2ilnd145opkgev)
Since budget is an issue, if you can wait until October there will be another
Police Auction October 29th. This is where I got one of my folding bikes for $80 bucks and I've seen others get a good bike for as low as $100.
I don't know what you're fitness level is. But if you're starting out take it easy, you really have to build up some fitness.