I will say: if it were me...

I would work up to the mileage first before making the trip. Unless the locking up and taking the train is a good option for you.

For that distance, I would seriously look at a road bike over a mountain bike. I dream many days of riding my racing bike instead of my mountain bike the 10 miles to work. Mostly just for time savings. I know, not a HUGE difference, but it sounds nice sometimes.

The back-up plan is a good thing to have, whether it is someone you can call to bail you out, the train, a cab, anything for "just in case".

Michael mentioned tools: new tube, patch kit, something to get the tire off the rim, and a pump. I carry co2 on my road bike, but a pump on my mtn bike. I have used the pump 3 times on one (ok, two at one time, but didn't know it) flat. First to try pumping up a seemingly slow leak, then two more to fix the two flats. That would be alot of co2 cartridges.

When I looked at starting to commute, I rode in on a weekend to see how long it took, and to try to find routes.

You will have to consider what you are going to carry your stuff in too. Backpack, paniers, ??...

That's a start anyway... Good luck!