Greetings from Paris!
If possible, I would strongly recommend that you cancel your hotel reservation and choose a hotel that is actually in Paris, preferably not too far from Montparnasse. Not only is your current hotel outside of Paris, it's also in an area that could be dangerous. On top of the overall high crime rate here, we are also experiencing civil unrest at the moment with violent riots taking place almost on a daily basis. It's pretty crazy.
I live in Paris and can highly recommend an affordable and friendly hotel near the Luxembourg gardens. There are certainly hotels closer to the Montparnasse train station, but this one is not too far and it's very easy to get to from either airport.
It's called
The Hotel Stella on rue Monsieur le Prince, 75006 Paris.
I think rooms are about €55 or so, but
click on the link above and contact the hotel if you're interested.
From the airport (CDG) just take the RER B (the only suburban train serving Paris from CDG) and get off at the station Luxembourg. Be sure to take an express train into Paris, you don't want the slow one.
Walking to the Gare Montparnasse from the Stella will take you through the beautiful Luxembourg Gardens. What a treat!
If you have any other questions don't hesitate to ask.