So you're climbing the Galibier from the Telegraph side... I wouldn't worry too much about Alpe Dhuez then because if you can make it up those two climbs then the Alpe really isn't all that bad. The Alpe has the infamy because it's a finishing climb and that's where the big moves are often made but the Galibier is far tougher. Distance - about 20miles including the small descent off the Telegraph, worse road surface, altitude, weather conditions and being exposed (no trees or surrounding shelter) on a majority of the climb unlike the Alpe. If the weather turns bad - it gets bad... very bad, very quickly. Jump on youtube and search for Pantani Galibier and you'll see what I mean.
There's only a couple of nasty bits on the Alpe, it's not really a difficult climb with moderate fitness and reasonably low weight.
Because of the severity of the climbs you'll want to try to lose as much weight as saftely possible and increase sustainable power on the climbs.
How much riding have you done before and if you don't mind me asking, how much do you weigh? That'd somewhat dictate what you need to do over the next 6 months or so. You'll need plenty of seat time, preferably in the hills with plenty of 8% to 10% gradients although shorter hills upto 12% would work. I'm being relative here - I'm not talking about piddly things that are 1/2 mile in length, unless that's all that you have available. Most of the climbs are at 6 to 8% but there are key points in that ride that are sustained 8 to 10% for more than a kilometer above 7,000ft You'll also want to get some very long rides in - 6 to 8 hours.
Get your position on the bike sorted months in advance if you're having issues.
Gearing. If you've never ridden in the high mountains before it's somewhat of a crapshoot. Some (a few) can get away with the gearing they use on regular hills and ride all day with no worries but you'll probably want to make sure that you have something pretty low. Not just a sprocket with two more teeth on. If you're ready for hitting those 10% sections near the top of the Galibier in driving freezing cold rain with 30mph winds when you're starting to crack/bonk because you've been climbing for 30+km then you'll have the right gearing. Chances are it won't be like that and your bottom gear will be used once - but if the event is outside the "window of nice weather" ie about 8 weeks starting in July then the weather can change from blue skies to a freezing hell on wheels in about 15 minutes. A wrong choice of gearing could make your ride a very grim affair.
Arnie Baker does a good book on riding The Death Ride - which is a similar event in the hills of Northern California's Sierra Nevadas. There's a bit more climbing and the distance is longer but all in all it's pretty much the same deal - 7+ hours in the saddle and 100+miles many of which are at high altitude. It's available in .pdf format and a quick search on google should get you to the webpage for that. It covers training, position, climbing/descending techniques, altitude and all the funstuff that goes with it and gearing....
You'll want to be fairly handy at descending as well. Not necessarily fast but comfortable in your ability to read the road ahead and relax and not use your brakes so much.
One thing you may appreciate when descending the Lautaret (you descend from the top of the Galibier to the junction at the top of the Lautaret) to the bottom of Alpe DHuez is a strong pair of wheels. The tunnels on the main road have (or had, it's been more than a few years since I was overthere) some very dubious sections of road surface and the tunnels are pretty long and dark. From what I remember I wouldn't descend down there at 40+ mph on a lightweight pair of carbon clinchers.
When you make that right turn to descend the Lautaret to Boirg dOsains be thankful that you are turning right and the organizers aint making you turn left so you can descend the other way and finish on the Col du Grannon. If that was the case I'd post this instead of the above post:
Have fun. Enjoy the ride. The Galibier, from Valoire, is one of the tougher hills in that region but if you get pretty light, train fairly well and choose sensible gears and pace yourself well on the climb then it could well be a very spectacular day. The views you'll get from the top of the Galibier will be something that you won't forget. Just don't spend too much time trying to take in the views as you descend
