While I don't know if Chicago has a cyclist ban on the marathon course, I can attest as a rest station volunteer for several years (between 2006-14) on the north and west sides that I never saw any cyclists ahead of the wheelchair/handcycle racers.
Also, the New York marathon is the first Sunday in November -- the same day that we fall back to standard time so it is light earlier in the day than the second Sunday in October. For Chicago, sunrise on those days is 7:00am in October and 6:26am. in November. The first wave of the Chicago marathon (men's wheelchair) starts at 7:20am and the runners at 7:30. Per the published marathon street closure listing, all relevant streets (away from the start/finish) are closed at 7am.not earlier.
Bear in mind that the first wheeled finishers are going about 17mph and the first runners are going about 12mph. A pre-marathon bike ride would likely need to be in the dark and pretty speedy.
Barbara