24 miles round trip is a long haul if you don't have much experience on a bike. It's totally doable, but you will want a bike that's properly fitted for you or it will be miserable. It might be a good idea to work your way up to it slowly by driving to work with the bike in the trunk, then biking home, then biking to work the next morning and driving home.
If you decide to go with a flat bar bike I'd strongly suggest some bar ends so you've got another position to put your hands when they start to get uncomfy. Being able to move hands around prevents them from getting as uncomfortable.
That bike seems OK for what you want. Wide gearing range is nice if your ride is hilly/windy. Rack attachment points so you don't need to wear your gear in a backpack (puts extra pressure on your wrists on a long ride). Yeah, add a bike rack to the shopping list. The tires are 28 mm wide, so a little lighter, but not as comfy as 32 or 35 mm ones . . . Might be something to consider changing if your ride is bumpy. The wheels are 32 spoked, which should be fine if you're lighter weight . . . If you're over 200 lbs though, I'd expect issues with the spokes within a couple years. V-brakes are fine (disks are nice in rain and snow).
Get some bright lights (rear - planet bike super flash turbo, cygolite hotshot, front - cygolight metro 350 or 500 are great) for dusk/rain riding as well as at night. Speaking of rain riding, it's REALLY worth getting a full set of fenders . . . protects your drive train and keeps you from tasting road every time it's damp out. You said back roads, how's the lighting on them? You may want to go with two rear lights and one of the super powerful 800+ lumens front lights if you need to light the way in pitch black conditions. Get reflective tape and put it all over the bike. Make sure you've got front and rear reflectors. Wear a bright coloured jacket. Put reflective tape on your helmet. Reflective ankle bands. You can't be too visible.
Bikes from Nashbar are fine (I have one that I like a lot) but you will want to take everything apart and grease the hubs/all bolts that go into it/seatpost/brake pivots as well as adjust the derailleurs and brakes once it's delivered. If you are terrified to work on bikes it's best to have a friend or local bike shop go over it.
Just my 2 cents. :P