SLOW DOWN! (sorry for shouting). Start with a run/ walk, gentle running , not so you are out of breath- you should be able to talk.
You have plenty time 'til october- you need to build a strong base of sloww mileage- get a HR monitor if you can, and check your max HR ( test it , don't calculate it), then try to stick at about 70% of max- this will be so slow you need to walk at times to keep rate down- don't worry, it will rapidly imporrve, and you won't injre yourself in the process. Going all out as fast as you can is the best way to disaster.
Look at the HADD thread, or search for base trainig.
Take it easy- the first run always feels horrible!