Fit is very important especially at the OP's distance.
I ride 45 miles each day and every small tweak helps. This morning I moved one shoe's cleat a small amount and it made an amazing difference.
Go slow to start and work your way up. I'm at the point where I have to hammer each day and ride 7 days a week to see any improvement.
I would also recommend Chris Carmichael's books. I'm reading one of his (the name escapes me) and an older book by Greg Lemond.
Be patient and you will get there also.