The best part about keeping something like this going is you will always be getting better at a rather decent rate. Just stick to it and force yourself, if you arnt ready to pass out at the end then you arnt running hard enough.
In Nov of last year i had a 24min two mile. As of this Nov i have a fastest run of 15:54 2 mile. one year, eight mins off, and 50 pounds lighter.
This is from doing almost exactly what your doing. Just keeping makeing it harder as you go.
However i do have to say check out some nice running shoes if you dont have a pair. The price is worth saving your legs. I picked up a pair of Brooks Pureflows 6 months ago and fell in love. They fixed all splints and are light as hell.