Hey Nic,
I'm planning a trip in 2 weeks to the MR! I'm giving it 3 nights (it's winter, and shit happens). I've never been to the mountain, so all my advice is unsubstantiated.
my 1st thought for you was acclimatization, but if you're hitting Rainier, Hood & Shasta immediately beforehand, I think maybe you'll be acclimatized.
Macnamara writes in his guidebook that , "a few motivated individuals" will do it in a long day. I'm sure you are motivated, but I think he is saying that it sucks to do it that way, like it's tiring & exhausting. 2-days would probably be more enjoyable (plus you get to wake up to a romantic alpenglow, high in the mountains with your main squeeze)
If you bring gear, know how to use it!
