Starting off with aerobics every other day is fine, you need time to build your fitness. Start with about 15-20 minutes a day while you find out how much you can handle.
Which pedometer are you getting? A strictly mechanical one is not very accurate as a small variation in stride length can throw it off by quite a bit.
Your estimate is assuming a 5 foot stride which is quite long and more of a running stride length.
"To give anything less than your best is to sacrifice the Gift."