actually, your cannondale quick 6 is probably a pretty good bike for long distance, thats a flat bar road bike right? honestly, it sounds like everything your doing right now is working fine. your obviously not a spring chicken and even though you may only be 4'11, 135 is certainly far from being morbidly obese. As you said yourself, you started out needing an hour to do 5 miles, and in a relatively short time your already doing 10 miles in that timespan. I think at this point just make it a point to keep getting out doing what your doing (buildling your aerobic base as others have called it) and don't really even concern yourself with truly "training" again, don't even think about mileage, the 500 mile base rule is a general one. my 500 miles is a lot different from your 500 miles. for me, even in bad shape, I can average 15mph pretty easily so i'll hit that 500 mile mark before my body has probably truly gotten an adequate cardio base.
Do you own a heart rate monitor? the one aspect of training you can do is a 30 minute time trial to figure out your lactate threshold. Then we can give you a better idea of where you should be looking to keep your heart rate while you do these so called "base miles"