IMHO, Apple botched this by releasing the 13" MacBook Pro non-touch early. Precisely because those are the only models being reviewed, and people are assuming so many things about the new model. For example, the fact that today's 13" and last years 13" have essentially the same CPU performance (WAY faster SSD performance this time around); has a lot of people reporting the same is true of the 15", when in fact, the 15" is a serious improvement over last years 15".
The 13" did not include an extension cable. The 13" touch and 15" touch may or may not. It's not unusual for Apple to include more accessories in a higher end model.
You can buy one, and yes, your old one will work. This is a standard that Apple has used for years. You can also use the power extension cable with an iPad brick, for example. Or you can unplug the extension cable from the MacBook Air/Pro brick, and plug in a two-prong from any of your iPad bricks, and have a nice portable charger that fits nicely in your bag.
I wouldn't go ordering an extension cord just yet until the 15" reviews come in and we see what's actually in them.