I'm currently training for marathon #4, I'm not the fastest runner, so you can take my advice or leave it.
This time I'm running T/W/Th/Sa/Su with full body weights on Monday and rest on Friday. Runs are roughly short/medium/short/medium/long.
This is the same as what I did for marathon #3, and shaved nearly 30 minutes off my time. But, like I said, I'm a slower runner, so 30 mins isn't as huge as it sounds. Last go around, I occasionally subbed in a circuit for the full body weights, depending on my mood. I did easy weights (PushPull/Supersets) for the last month just to lessen the risk of injury or doing something stupid to myself so close to race day.
The 1st two marathons I did, were running on T/W/Th/Sa and then I would try to stick in either 2 upper/lower splits, normally lower on Monday and upper on Friday and non-running cardio on Sunday. That was a lot to do in a week, especially as the runs got longer. When I trained for my first, I was also wrapping up the last couple weeks of STS, but at least the runs were shorter.
I've discovered that as the miles get longer, I have to have the rest day, and by trying to cram too much in and do it all, I end up worn out and cranky. The problem is, there is so much I want to do!!!!
Nan