Don't start in the deep end. 20 miles RT is a fair amount. Mine is 18. I started with the same mentality and I had to push through a pretty painful stretch. You may hit the same wall and want to quit. 3 times a week to start should be plenty. I didn't, and that wasn't the brightest move on my part.

I like the reccomendations already on buying, but for that distance I'd encourage more of a road geometry where you can get out of the wind and aren't completely upright. I'd also focus on safety gear (visibility vest, helmet) first. Since you will likely need to change clothes, I also wouldn't worry about fenders right away. I personally don't use them.

(PS I ride a cross bike from BD. I think you get good value there)