I'm not familiar with the dvd you're using, so you'll probably get some better answers than mine, but here's my two cents.
It depends on your goals. If you want to build endurance and your cardio ability, then continuing to build up walking those two miles at a quicker pace would be better for that. If you're strictly looking at calories burned, then in general that's independent of your speed. Two miles is two miles, and it's the distance that matters, not the speed, for the most part.
That said, I have seen a non-scientific, informal study that showed walking fast or running slow burned the most calories per unit distance. This was attributed to the fact that when you're walking fast, jogging would be more efficient, and when you're running quite slow, walking would be more efficient. Again, it was not a scientific study.
Sorry no sources cited...I'm just recalling things I've heard in the past.
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