Seriously, a 15 mile trip would get you into shape, but not make you look like a stud muffin or make you loose and weight. Not like you have to.

A always go for used. A good road bike frame on craigslist. Make sure it fits you and research the bike around the net for the pricing. I can usually find a 2 year old trek for $300-500 and a older 10 speed for $100-200. I would build a bike for city commuting, single speed with everything you need. riding your 29er is good for a time, and good for a back up, but make sure your going to lock it up where you're not going to lose it. I ride a Trek 220 girls bike to work because no one would steal it rather then my road bike with $700 into it.

My plan for you:
1.Find out about parking your bike, or can you have it placed inside with your job?
2.Ride 29er to work, lock with loose cable and u-lock
3.Build roadbike, prefer singlespeed.
4.Collect helmet, safety vest, and light if at night. You're a grown man on a bike in heavy traffic, just add a couple more embarrassing parts and wear something for safety. And any racks and panniers you might need.
5.Learn traffic rules and regulations
6.Learn about bike routes and less traffic areas to in your commute.
7.Start riding the built road bike.