There are some ok hotels in the Lincoln Park/Wrigleyville area as well as in the U of I area. Not sre what their availability is. Club Quarters has a $209 rate for that weekend and by Chicago standards for that weekend, it's a steal!
Don't know if you've been to Chicago for the marathon before, but the last thing you want to be doing that morning is fighting th traffic to get into the city. With the number of streets closed and Chicago traffic in general, it's a mess. So, if you can find something close to an L station, preferably the Brown or Red lines you'd be best off. Those lines are also the best for enjoying the city either before or after.
There are only a few B&Bs in the Gold Coast/Mag Mile area, but they are pretty nice and darn convenient.