Since you have been riding 6 mile stretches already, it should not take long to get used to the 26 mile commute (13x2)... perhaps one to two weeks. I suggest riding just 2 times on the first week then increasing the frequency the next week.

Before your first commute, bring your clothes, shoes, etc. to the office in advance on your car so you don't have to carry them around while you are building up strength.

You should at least learn how to replace an inner tube in case your tire suffers a puncture. I always carry the following items with me when commuting:

1) Tire levers.
2) A small frame pump.
3) An Inner Tube.
4) Tube Patch kit.

These items except the frame pump can fit in a saddle bag (the bag you attache beneath the saddle).

It also helps to have a multi-tool with hex wrenches and screwdrivers. Once my saddle loosened up during my commute and I was able to tighten it because I had the multi-tool.