You can't just start off riding 40 miles a day. How much cycling experience do you have? As commutes go, that is enormously long. I found that just the shear time it took for me to do a 15 mile one way commute (about 50 minutes one way) wore on me. I was doing it every day, but I am also a pretty avid cyclist and ride a lot anyways. I don't want to be Debbie Downer here but it sounds like you really don't have the necessary fitness or experience to be doing this. If you are committed to eventually being able to do this distance, I would get some sort of road, cyclocross or touring bike, with a good gear range and drop bars. Then start by riding for recreation and shorter errands. Also, once you do decide to commute by bike, do a trial run or two on the weekends to get an idea of how long it will take you.
I'm sure some people will disagree.
Somewhere either in this thread or another the OP mentioned something about 3 days/week. That's a lot different than trying to do the ride 5 consecutive days.