Dont make running complicated with elipticals and all of that other fancy BS. Just run.
Honestly I think a lot of people think too far into running when it is actually the most simple sport out there in terms of what to do to improve; run.
Im not a big fan of running indoors either, I always viewed it as wimping out. I ran everything from the 400 meters through the half marathon at very competetive levels (NCAA championships), and never once did a run indoors. I live in Chicago too, so I experienced everything from -30 windchills to over 100, and ran them all outside. It toughens you up not so much physically, but psychologically when you do a hard ass run in the heat. Im not advocating being stupid about it and having heat strokes because you have no water, but I hope you know what I mean. If the weather is really extreme, Id suggest doing a couple short loops around your home so if it gets really bad out, or you start feeling funny, you can get back to your home relatively quickly.
If you start running on treadmills indoors a lot, when race day comes and its warm out (or cold) you may shut down psychologically because its in conditions where you would have run inside or done an eliptical workout...I have seen it happen. Train in all weather, you never know what it will be like on race day.