Pierson, IL
West Frankfort, IL
Anadarko, OK ... just west-ish of Oklahoma City, OK
Gilman, Illinois just north of 24!
By road, for practical purposes, Indianapolis.
Indianapolis
A city halfway between Billings MT & Chicago IL could be Sioux Falls SD, depending on your route.
The halfway point between Coldwater, Michigan, and Springfield, Virginia, is on I-76 (Pennsylvania Turnpike) where it meets I-376, just northwest of Pittsburgh and southeast of Youngstown.
About 45 minutes to 1 hr, and the distance between Naperville and Chicago in a straight line is 26 miles or 41.83 Kilometers.
It is 145 miles between Indianapolis, IN and Mattoon, IL.
The halfway point between Logan, WV, and Bloomington, IL, is roughly near the area of Indianapolis, IN. This city is a convenient stopping point as it is approximately equidistant from both locations. Depending on your specific route, you might consider stopping in or around Indianapolis for a break.
When taking Interstate 65, it is near exit 193 in Illinois, the interchange with U.S. Route 231.