I think the best thing to do is go by feel. I am not a big advocate on distance until you are fit enough to go certain distances and even then you might over due it. Then wind up with an injury. My best advice would be to visit a local track and run 400m walk 400m etc.. until you feel up to running a full mile. Then start by running a mile and walking 400m. Nothing wrong with walking in my book to keep you pushing for your goal. I'd also pick up a running book and look thru it for some sort of training plan. You get faster, like 1748357 said, by doing sprints or fartleks. Listen to your body and you should be good.

You should easily be able to get your body in shape for the duathlon well before the fall but maintain the fitness so you are prepared for the duathlon. If I knew more about your age, weight, height etc.. I might be able to have a better prediction for you. I can typically do that distance without training now. I'd suffer through it but I mentally know I can do it so I just do it. When I first got into multi-sport events I wasn't too sure I could do something like that and prepared a lot for the events. Now I just wing it. My way is not a good way it's just what has happened to me due to time restraints. Everyones different and the most important thing is to have fun while living a healthier more fit life!