I'd say keep your old mountain bike around. Does it have suspension? If not that's even better (on flat and more or less smooth surfaces like paved roads suspension just adds weight and the suspension absorbs some of the energy you're trying to direct into your pedals rather than putting it all into forward motion, but it's not a deal killer if you have it). Online, you should be able to get slick tires for it for ~$10 each that will help a lot in reducing the effort required to pedal (low pressure knobbed tires take more effort to pedal on a paved surface than smooth higher pressure tires). A cheap second rack can be had for another ~$20, and add in some fenders for a total of about what you're saving in gas for one week. Presto and you've got a backup bike in case something goes wonky on the Denali and, more importantly, it's perfectly set up for riding in on those rainy days when your wife isn't able to drop you off due to her schedule but you still have to get in. Throw on a rain cape and you'll be more or less dry when you get to work. I know that extra $60 may be more than you want to spend right now and having a second bike may seem dumb (I haven't been able to convince my wife I need one yet, so my only bike has these modifications and more on an old mountain bike and is heavy and slow), but all I'm saying is keep it in mind as you continue riding and don't get rid of that old bike if you've got room to keep it.