I agree with most of what has been said here. However, don't be discouraged; with a good guide and intelligent acclimatization, you could still get Cotopaxi and/or Chimborazo in before you leave, though with only twelve days in country, it might be an unpleasant journey. As was said earlier, doing a 4000m peak (Fuya Fuya was suggested earlier, I second this recomendation) then Iliniza Norte, followed by Cotopaxi then Chimborazo would be the reccomended order of events.
A bit more about each of these peaks. Fuya Fuya is easy, there is actually a road all the way to the top. Take a bus heading towards Otovalo, get out at Tabacundo and ask anyone. They'll point the road out to you. A fun trip is to walk up one side of the hill from Tabacundo, camp near Las Lagunas de Mojanda at the top, then walk down the other side into Otovalo.
For Iliniza Norte, take a ride to El Chaupi and ask for Sr. Vladamir, he runs a hostel in the next town up the road. There is a small restaurant just off the side of the road at El Chaupi, and the proprietors know everyone headed that way and will be willing to flag a ride down for you. Vladamir's hostel is very nice, inexpensive, and he will give you a ride to the trailhead in the morning if you want.
For Cotopaxi, there are a number of good guides in the area, Moggely is a very good one, in fact, he runs all of the rescues on Cotopaxi and Chimborazo, so he's pretty experienced.
If you have an option on Chimborazo, make it without the stop at the first refuge. Start up from the bottom alpine early, make it to the second refuge in one go, then summit and back down on the next day. I personally found the extra day at the first refuge to be disastrous to my health.
Also, if you are looking for english speaking company, at kilometer 25 on the pan-am, just short of tabacundo is a road with a sign that says 'Picalqui' walk down this road a quarter of a mile and take a left. You will be walking onto la fundacion de bretheren y unida, or FBU, run by a friend of mine named Stuart. Actually if you are going to stop by there, let me know, I owe him some stuff.
Best of luck, and enjoy Ecuador, its fantastic.