I totally agree with all the comments about fit of the bike being far more important than the material. I have a women specific Trek that fits me really well, and I could comfortably ride it all day. It's an alu frame with carbon forks.
Regarding training for the ride, you'll probably find your running fitness converts well, especially if you regularly run up and down hills. Converting from cycling to running is hard because running uses more muscles than cycling, but as a runner you'll already be using pretty much all of the muscles you'll use as a cyclist. The most likely problems you'll encounter to begin with are a sore backside while you adjust to spending hours in the saddle, and possibly some upper body stiffness from spending a long time in a fairly fixed position. Add some upper body stretches to your normal stretching routine.
The type of bike you choose is very personal, but my choice would be a road bike with drop bars. I like it because it goes fast for relatively little effort, and there are lots of different places to put your hands. This helps prevent discomfort from pressure on a certain part of your hand, but also helps to reduce the stiffness I already mentioned. Moving your hands changes your upper body position slightly and helps to prevent you getting "locked" in one position.