Wow, I can't believe I got 2 jerk responses. That's so far off what traveling is all about, which is largely about being open minded to and willing to learn from others different than yourself. I didn't think rude people like that would be cruising these forums. Nevertheless, thanks to the majority of you for your thoughtful and helpful responses. And for the record, the countries I've been to: England, Belgium, Netherlands, Luxemburg, France, Germany, Poland, Slovakia, Slovenia, Austria, Hungary, Croatia, Italy, Spain, Switzerland, Turkey, Iran, Thailand, Malaysia, Singapore, Japan, Canada, Mexico, USA.....24 countries, first time I've ever counted....my sincerest apologies for my huge embellishment.
I for one will not give your claim credence 'till you post the photo of the cute Tyrolan hat ...with a pin for each country visited...carracar

You're American and well traveled, so you're OK. On the other hand, it's open season on Canadian tourists this year. Don't get mistaken for one of them: tip really well, no maple leaf on your pack, don't say "eh".
Lots of people do the kind of travel you are contemplating, and if Mexico City is your destination, there are lots of routes that will get you there.

I don't see anything problematic traveling by land from the border; I haven't done that from San Diego, but I've done it from Texas. How much time do you have to travel? The answer to this question will help determine suggestions, IMO. What are your interests, your budget, etc., etc. You really haven't told us much, from what I'm reading. I'll second (or third) a suggestion that you spend some time at one of the large bookstores in San Diego . . . so that you can formulate a plan, and then come back here to present it and ask for refinement. Traveling alone shouldn't present any particular problems, either - though women might have some greater concerns than men; you'll encounter many solo travelers along the way.

In 2001 I partnered with a friend of a friend and we drove (his car) from San Diego on a two week trip down into Baja to Cabo. About three days into it, we got into a huge fight and parted ways just south of Mulege. I was on my own and still wanted to get down to Cabo so I hitchhiked (female) on my own the rest of the way, stopping at various towns along the way.
I had no problems and that's with the element of danger from hitchhiking. If you went down through Baja, there's a ferry that goes across the Sea of Cortes from La Paz to somewhere on the mainland and then you could work your way down from there. The bus system is very good in Mexico and you shouldn't have any problems. I'd think about pairing up with someone when you got further south, like Chiapas, to go across into Central America.
Fellow TTrs before you spend any useful time on this fool.... I"ll share a private message in my box today...carracar If you have nothing constructive to say in the future, most people on this forum probably would appreciate you keeping your unkind comments to yourself. While thats real cute that you operate a hostel and like fixing appliances, you are talking to a person with a doctorate level degree....something a person with your intelligence level would only attain in their most far fetched dreams. Your unconstructive comments couldn't be farther from the traveler's spirit of open mindedness and kindness. Being well traveled doesn't mean being a jerk to anybody who doesn't know something you do, people's curiosity for other cultures and places should be nurtured.....not met with rude criticism.

I don't understand the attitudes towards the OP. If you don't like his post, why not just ignore it and move on?

Wow, this has turned into quite a string. Thanks so much to everybody for taking the time to read through and offer advice. As to address a couple things people said. I'm looking to do probably 2-3 months of traveling. I'm debating on whether to do mexico/c. america or do south america. Mexico is obviously so close...but something about places farther away always seem more intriguing to me too. So thats what I'm deciding on. I'd definately like to take some spanish classes either in mexico or c. america to polish off the rust of my high school spanish. I definately will be spending a couple days in borders sometime soon reading thru all the different books, I do that before all my trips. Thanks for suggesting it, I know I have much research left to do. My budget doesn't have to be super cheap necesarily, but to be honest I like it better traveling cheaply. If I wanted everything always nice, a/c'd, and convenient....I'd just stay in the usa. Traveling cheaply brings you across more people, culture, locals, etc....especially when traveling solo, in my humble opinion.
As for carracar....he apparently wasn't hugged enough as a child....perhaps I'll pick you up a self help book when I'm at borders. Send me the address of your trailer park and I'll have it mailed directly.