Maybe I'm crazy for saying this, but I thought you were doing great considering your weight, age and riding a mountain bike!! Buy a road bike and watch your average speed go up 3 maybe 4 mph. I think you are a very strong rider, just keep practicing. As far as money goes towards a purchase of a road bike just keep your eye on Craigslist, Goodwill type of places, and garage sales, you can find all sorts of deals but you have to respond fast, and make sure you test ride before buying. Any older 80's lugged steel mid to high end Japanese made road bike with 36 spoke rims are very strong and will hold your weight with no problem and their fast, and with constant searching you can find them very lightly used for under $250. Then after several years if you want you can invest in a new bike, personally I don't care much for newer bikes especially CF and AL road bikes, but that's just my opinion not shared by many so take it for a grain of salt.
If you do go used first go to this site to check on the brand: http://www.sheldonbrown.com/vrbn-a-f.html
then after that simply do a web search for year, brand and model and sometimes a web site with the makers catalog will appear and you can check the catalog to see where that model fits into the level of bikes offered by that manufacture that year. Others will show forums of owners and will provide an indication of level of bike, some may be a site that just talks about it and are very informative. Posting here is great too...but by the time you get a response the bike will be gone. Level of components is a good judge of the bike's level. Familiarize yourself with the older 80's era brands like Campy (though difficult to find these on Craigslist and not typically found on Japanese bikes), Suntour, and Shimano groupsets, this will tip you off real fast as to how good the bike is; this info is available on the web.
I've purchased the following bikes for $100 or less on Craigslists over the past 2 years and their all mid level racing bikes from the 80's: 86 Nishiki Olympic with Shimano SIS 105; 85 Schwinn LeTour Luxe with Suntour Mountech (a touring bike but very strong bike and can be made to be a decent fast road bike with narrower rims and tires, but I'm using it for touring); and a 84 Fuji Club with Suntour ARX (newer models have the better Suntour Cyclone system). The Nishiki was in average condition, the others are in mint condition with less then 250 miles! Their out there, hanging in peoples garages for years after only riding sometimes for one season!
Maybe you have no desire to go that direction I mentioned, but I just mentioned in case.
Last edited by rekmeyata; 09-14-11 at 12:49 PM.