Linda,
I ran the Paris Marathon a few years ago and I think that it should pose no problems to you. The start/finish line is at the Arc de Triomphe, so I would avoid this area on race day and hotels near to the arc would be prime locations for marathoners. If I remember correctly, the race starts at approximately 9 am on Sunday, I would avoid the metro lines that lead to the Arc from 7:30 am to 9 am, better yet, just avoid the metro altogether between these times. Much of the race is in the Bois de Vincennes. I do not remember any portion of the race going into the left bank, so you would not even know there is a marathon going on. I would avoid the Louvre as the race makes multiple passes at the beginning and end of the race. I do want to state the obvious and mention that the evening before the race should not be the one night to be looking for Italian food, because marathoners will be carbo loading. However, you will be in Paris, and I doubt that you will be looking for Italian restaurants. Expect a lot of fit looking people who will be moving slowly on Sunday evening and Monday especially get off buses and down stairs.
But as a marathoner, I would embrace the opportunity to watch part of the race. It is one of the largest in Europe and I always find it inspiring.