Touring - Minnesota to Mexico

I'm going to bike from minneapolis, minnesota to mexico this summer but I've run into a couple problems. First of all I have no idea how long this will take, I want to say 1 month, but this seems absurdly short for going across the US. I'm wondering if anyone has done a trip like this, and, how long you think it would take. is 12 hours a day reasonable? are there any computer generated routes that exclude freeways and highways? so i dont have to sit over a map for 10 hours trying to find the best route.

Also, since this is actually the first actual multi-day trip I'm going on, I'm going to need to get a new bike. My current bike is a 45lbs Giant DH Team (http://www.giantbicycles.com/us/030.000.000/030.000.000.asp?year=2002&model=10455) ... so I'm wondering what types/brands/actual road bikes are good for this type of ride that are under $1000 for a 260 lbs man.

Currently I am super anti-spandex, and things simaler to it, I just cant stand seeing myself in a skin tight suit, and im sure the people around my would rip they're eyes out if the did see me in one. Actually its not quite that bad, but i do like to be confident, and spandex = poor self image = depresion. So im wondering what would be a good alternitave for biking atire, would normal clothes be fine? shortsleeve shirt, kaki shorts? or should i go for a jersy?

Also, please dont just tell me that im not ready for this, this has been my goal since 8th grade, to bike to mexico before I graduate, and this is my last year to do it. I do bike whenever i can on my mountain bike, so i figure that might give me a slight advantage in starting up, concidering its like always pedling in sand. Even so i do know that this is going to be a huge challenge, and I accept that.

Oh yeah, whats a good mexico city to cross over, thats fairly close, apposed to the western edge? concidering I am going to feel like death at the end of it all (hope not, but might as well plan for it). :D
I guess it will be a good story either way, I biked to mexico, or, I fell into a 3 month coma 1 week into my trip to mexico.

I met a guy two years ago that rode from Winnepeg to Brownsville TX. IIRC He said he mostly followed US routes 75 and 77. If I can find his email address I'll PM it to you.
From Minneapolis it looks like about 1700 miles to Brownsville. Assuming you are in decent shape when you leave 50-60 miles a day should be no problem for you which would get you there in about a month or so.
A 45# DH bike probaly wouldn't be anyones first choice for a touring bike you might want to find yourself a touring bike or even an older rigid MTB.
The spandex looks goofy but it does work best at keeping your crotch dry and not getting sores. My azz is pretty well work hardened from years of riding and I ride a touring bike with a MTB bar and a brooks 66 seat so I can get away with just cargo shorts and briefs . Several bike companies sell cycling underwear which is padded to wick away the moisture and avoid sores. You just wear them under regular shorts. There are also mountain biking shorts that are more baggy and have a padded innerliner. Jerseys are preferred for comfort but T-shirts are ok too I usually buy the longer cut ones like Champion or Jerzees brand. Check around at the various online stores for cycling clothing, Not all cycling clothing looks like camoflage for a gay bar.

Your trip is certainly doable but you need to get busy getting a route together and figuring out where you are going to stay each night.