I have cycle toured a bunch and been a more-or-less daily bike commuter for the last 10 years. I do my commuting on a beater MTB because it's short (4-5 miles), and there are a couple of places where I need to drop off curbs. I also have a punch-me-in the face road bike for summer road biking.
I'm seeing a range of suggestions above, but I have to say I would agree with the folks who are suggesting a bike with dropped handlebars. The position is more efficient, both anatomically (you get more power with a more forward position), and aerodynamically, and I expect you would see a really significant difference in the speed you can maintain. Moreover, a more road-focused bike will have a lighter frame, which also makes a difference. Put another way, it's pretty normal for me to be passed a couple of times when I'm commuting on my MTB, but I'm almost never passed by other cyclists when I commute on my road bike. I'm substantially faster on the road bike at the same level of fitness/effort.
As far as what bike you choose, there are a range of options, from "hit me in the face with a blunt object" to "a light slap with a glove will suffice." Getting a used road bike from Craigslist could be pretty economical - probably less than what you paid for your Marin. The only challenge is that at your height, it could be a long wait to find a frame that's the right size. Or, you could buy a new bike at pretty much any price point you want. You could consider a cyclo cross bike like the Surly Cross-Check that others have suggested -- that's a well-regarded bike. My coworker commutes on another cross bike, the Redline Conquest, that she's happy with. Or you could consider a road touring bike - the Trek 520, REI Novara Randonee (especially if you use the member's 20% off coupon), or similar. Compared to a cyclocross bike, which is more competition oriented, a road touring bike will be more stable and offer better heel clearance for panniers, but heavier and less fun to ride.
If you're buying new, I would stay away from buying a pure road bike - as noted by other commenters, they will not allow you to readily attach racks and fenders. While there are workarounds, they come with weight limits (i.e., "floating" racks that only attach to the seatpost are rated for a max of 20 lbs, vs. 50 lbs for a traditional rack) that make them impractical for year-round commuting. An older used road bike is more likely to have attachments for racks and fenders - check before buying.
Agreed that the sore butt will go away with more hours in the saddle. Usually the 3rd day of extended riding is the worst, and after that you'll be on the mend.
One afterthought - BikesDirect.com is a possible source. They basically buy generic frames from Chinese/Taiwanese factories, put nice components on them, and sell them for a great price. Do your research before moving ahead - they get both good and bad reviews. If you do decide that you're interested, this bike could work for you:
http://www.bikesdirect.com/products/gravity/liberty_cxd.htm