another thing you most likely havnt thought of is the heat generated by running on a treadmill.
I hired one for a month in my old flat at uni - i had a HUGE bedroom and plenty of space for the thing, but when running on it I was pretty uncomfortable as my room got very hot very fast with it, even with windows wide open, there wasnt too much airflow which made it so much more effort. To be honest the gyms iv been to have been bad for airflow too...
Best bet is run outside if you can, its more enjoyable and you can speed up a little when you want to cool down.