The Great Western Trail starts on the west end of St. Charles IL and runs west out to Sycamore. I've never run it all, but I've gone out over 10 miles and not run out of trail. It's crushed limestone with a couple miles of blacktop in the middle. Very few streets to cross and it's mostly wooded on both sides. Mostly flat but you have a couple bridges to cross. It's used by horse riders in the summer so watch your step, but I've seen deer while running there too.
Map - http://www.co.kane.il.us/Forest/gwtrail.htm