KR - Something I've started doing in the past few months is simple weights 3 times a week in my lunchhour. Nothing fancy, just some simple compound moves like bench press, deadlift, overhead press, squat along with incline flys, some cable stuff,etc. It fits in nicely with 50mpw without affecting my running and (I feel) complements it well. Steve is spot on with saying 15mins at home is doable - press ups, reverse leg curls, planks, side planks, tricep dips off one of the dinner chairs. There's a tonne of stuff you could do.
You're trying to "injury proof" your body as much as possible and make sure that you don't fall apart in the closing stages of the marathon