Pack voltage will only affect top speed; although you can always compensate for that with a higher KV motor. ESCs are usually most efficient running at a voltage close to their max rating, so on a vesc 6 based ESC you would want 12s (or lower), and on a cheap vesc 4 based ESC you would want 10s (or lower).
Sounds legit for a slow single drive; my friend gets 5-7Wh per km riding slow on urethane wheels, my brother gets 7-15Wh per km riding at a medium speed, and I get 24Wh per km riding at full throttle. 
You should look at some of the other “first build” threads to read advice given to others; feels like I’m repeating myself here, but the order in which you should plan parts for your build is the following:
Once you have an enclosure and deck in mind, then you can figure out what layout battery will fit in there.
Btw, assuming the cell you modeled your pack stats over was a 30Q, according to Mooch’s testing, from full charge to 3.2V at 10A draw, each cell only provides 7.5Wh of power, so a 13s2p would only give you 195Wh at 20A total draw, not 280 