If you were doing this route on the flat, via the Maurienne valley, it would be a tough day in the saddle but in Acte I you are crossing two hors categorie 2000 meter mountain passes: the Madeleine and Croix de Fer with the Glandon thrown in for free and the tough but short 6 km climb to the col de Mollard and first category climb to the ski resort of la Toussuire to finish you off.
The route is shortish, 152 km in total but covers 4900 meters of vertical. Apart from the first 20km from Albertville there is virtually no flat on the course. You tackle the climbs and descents one after another. If you are not used to that type of riding it can have a serious impact with a big change in pedal cadence and effort. Going straight into a climb after 30 minutes or more of descent while hardly pedaling can leave you hitting a wall.
You also need to be able to feed and drink on the move, either while climbing or during the fast descents. Riders lacking descending skills may be in trouble, too scared to reach for a drinks bottle or food in their pockets they won't recuperate enough for the climbs.
You will need to train on varied terrain, passing from 53x12 (or 50x11) to 39x25 (34x25). You can do this on a turbo trainer as well. Practice taking food and drink on fast descents. An exercise is to take food from your pocket with your left or right hand, pass to the opposite hand and put it back in your pocket, without losing a crumb. Relax your muscles on descents, it will be your only respite on this stage.
Longer than Acte I: 201 km and 5000 meters of climbing with 4 cols plus the Col de Soulor for free on the descent of the Aubisque with a finish after 15 km of descent to Bagneres de Luchon.
The first 30km are a gentle climb to warm the body. Afterwards the difficulties are progressive, giving you a chance to ride yourself into the stage. The only difficulty to this plan is the Col de l'Aubisque, tackled first, is also the steepest. Afterwards all the climbs are tackled by their easy sides. The cols are linked by routes along valley floors, giving the rider a chance to recover and spin their legs. In reality the finish is at 185 km as you can simply roll down the final 15km, but watch the road and other riders who will be tired as well.
For Acte II work on endurance. If you live in the flat you should have tackled a ride of at least 250 km in the weeks prior to the stage. Think about adding the KM onto another sportif. You will need to take care to drink and eat enough along the route to avoid the bonk. Check the weather, in the Pyrenees there can be big changes between each valley, make sure you are warm enough for the descents.
In conclusion don't think the shorter stage is necessarily the easiest. There is less dead time, good riders will climb strongly. Try and keep up with the group ahead, at the summit of a col or at the bottom of the descent make an effort to get on the back of that group, it can give you a margin later on. However do not continue alone in the valley, you will tire yourself out and will be caught by the group behind on the next climb. Wait for them.
http://www.letapedutour.com/ - Le Etape du Tour Official Website
http://www.velo101.com/videos/voir/224/reconnaissance-etape-du-tour-2012-acte-i - Video review of Acte I, link in French