I did the Baltimore half marathon in 2008. The half marathoners and full marathoners join together at mile 15 of the marathon course and the two courses are the same after that. Be sure to check out the elevation chart. The route has a big uphill climb from mile 18 to mile 23, and it's known to be a hilly course. Be sure to do some hill training.
There are also several supported course runs that are particularly geared to marathoners, and if you get in touch with local running clubs in Howard County, Baltimore, and Anne Arundel county, you'll be able to get in contact with others who can give you feedback on the two races. The race bulletin board on the race web site is also a good source of information.