Wendy - You can book whenever you like really. If you want to stay near the Arc start/finish area then it's best to book early. Lot's of people here use airbnb to book apartments. The Hotel Astrid is a popular one for people who want a hotel near the Arc.
I've always done it on the cheap and stayed at the Ibis in Montmartre, which is a 15/20 minute metro ride to the start. The good thing about Paris is as long as you stay somewhere near a Metro then it's easy to get around. Metro lines 1 & 2 certainly have direct access to the start/finish area.
Well to be honest due to post Loch Ness CBA and then buggering off on holiday for most of November I had the best part of two months of no running. I think I ran about three times between LN and starting training for Paris in December.
HiaHS - I hope you're back to form soon. On a positive vibe I was in horrifically bad running condition at the start of January and I've gone on to smash PB's all over the place this year from that poor starting point. It can be done.