Well done TP, OO.
SR - Races added. Any more for any more?
Holey Moley - In answer to your earlier questions. My answer, and there'll be 50 different answers, is that you can replace the steady run with a treadmill session...but it's not like for like. Ten miles on a treadmill isn't the same as ten miles outdoors. I avoid treadmills where possible as I detest the monotony of them, so I don't know much more than that, but someone here will have a much better idea of what the differences are.
Regarding your other question, I'd say if you have a spare four weeks, it would be better spent building up your base fitness rather than using it on doing more long runs, not because of the risk of injury, but because base fitness will help you get through the long runs and marathon in better shape. Having said that, everyone's different, so what works for one person might not work for another. I'll do stuff that other people here don't and vice-versa, so I'd get a few second opinions and then decide.
One thing I'd recommend is keeping a diary of what you've done so you can look back and see how your pace etc has improved over time. And it'll be invaluable if you decide to do another marathon after Paris as you'll be able to look back, see what you felt worked, what didn't work and where your strengths and weaknesses are, then tweak the plan accordingly.