Welcome to active.com!
I think it's great that you want to include strength training as part of your marathon preparation. Far too many distance runners neglect their upper body conditioning and that is a big mistake. Tired arms, shoulders, back, etc. lead to poor form at the end of longer races. The key is to participate in a routine that utilizes high repetitions of relatively low resistance. You want build endurance, not bulk. I like to do natural resistance exercises (ones that utilize your own body weight - push-ups, pull-ups, crunches, dips, etc.) because you can do them pretty much anywhere and anytime, but weights are fine if done properly. Here's what I do:
2 or 3x week - 3 sets pull-ups, 3 sets chin-ups, 3 sets dips, 3 sets leg raises
6x week - blocks of 3 sets push-ups, 3 sets crunches, and 3 sets bicycles done once in the morning and once in the afternoon/evening
I hope you find this useful. Let us know if you have any more questions.
Good luck and happy running!
Jay

Here's a good article from Runner's World that describes some good core exercises:
http://www.runnersworld.com/article/0,7120,s6-238-263--11400-1-1-2,00.html

Here are some other ideas:
Spacing your hands at various distances while doing push-ups will work different muscle groups, as will elevating your feet.
You can do a modified version of dips using a stable chair.
There are some fairly cheap pull-up bars you can buy to hang in a doorway (or you can go to a local playground that has a bar set).

Happy running!
Jay