12-15 miles = 20-25km = quite a bit for starting out, probably 45min each way? Not surprising that you're tired and sore. Make sure you get your rest as well as you exercise, just imagine, you're doing 10 gym sessions per week!
My suggestions (same as everyone else!):
(1) Check bike fit, saddle, drive train (chain clean, brakes not rubbing) and get 1-2 rest days per week; eat enough and drink enough.
(2) If that is taken care of, things will get easier after 6 weeks or so, then you will probably plateau. By this stage you should be reasonably comfortable and able to complete your commute with enough energy to get through the day and home again. If you cycle for transport only, you should now be fine.
(3) If at that stage you want to improve fitness/speed/distance then check (1) above, if still OK do some focussed training.
Stick with it, maybe take a rest day or two in the middle of the week if you have alternative transport available. When I was doing my "long" commute I got really worn out if I did it more than 3x/week.