Hey you're doing great! Keep it up.

My suggestion would be to wait a minute on the electric assist. If you're already athletic, you should get into good riding shape before long.

If your back hurts, check the fit of the bike. The saddle position is most likely to be the culprit. Make sure it's level and at the proper height. Then try sliding it forward and backwards, just 1/4 inch at a time, to see if you can find a more comfortable position. Keep the Allen wrench handy, and just keep adjusting as you ride. (Well, not literally as you ride -- pull over to a safe location first!)