Commuting to work is great, but you should probably try riding on weekends first and work up to a 15 mile ride. Unless you live in a flat area and/or are already in good shape, 30 miles a day four days a week is going to be extremely difficult.
I started commuting about 3 years ago. I'd had bikes on and off since I was a teen, so I picked up a bike off Craigslist, did some short rides on weekends, and then started commuting the 10 miles to work 2 or 3 days a week. It took a while to work up to 4 days a week and longer mileage.