From Montparnasse you can get the high-speed TGV train to a destination in the Loire Valley (ie Tours or Angers for under 50 euros) either as your final destination or to pick up your hire car for further exploration. You can also catch a standard train over to Chartres. Your ticket can be purchased along with your Eurostar ticket or separately - it should be less expensive (as little as 20 euros) the earlier you purchase it.
When you enter the station from street level you'll find escalators and stairs leading up to the main stations plus access to the metro, lines 4, 6, 12 and 13.
Journey times from Paris Montparnasse (TGV) station.
Tours: (**Saint Pierre des Corps):1hour 15 mins. on the high-speed TGV train
Angers: 1 hour 40 minutes
Blois: 1 hour 35 minutes
Bourges: 1 hour 50 minutes
Chateauroux: 2 hours 10 minutes
Chinon: 2hr / 2hr 30min
Orléans: 1 hour
Chartres: 1 hour 15 minutes by TER train
Chatellerault: 1 hour 30 minutes
Poitiers: 1 hour 40 minutes
**Saint Pierre des Corps is just outside the city of Tours. If you are heading into Tours itself you will find a non TGV train waiting to take you there on your arrival. You can also pick up a hire car from here for onward journey to you destination.
Although Chatellerault and Poitiers are not in the Loire Valley they provide good links into the Southern Touraine area of the Centre-Val de Loire region.
You can pick up a standard commuter train from any of the above stations to other destinations within the Loire Valley, for example to Loches you would transfer to another train at Tours.