You'll be much happier at cadence of 80! It's not clear what a "long hill" means in terms of length and grade, but there's nothing wrong with stopping if you need to. I used to do 9 miles each way on my commute, but it took going for 25+ mile rides for me to see improvement in endurance. 15 mile rides are a great start! You'll be able over time to see improvement in your fitness with the long hill. I have been through similar riding situations and still have a terrible time with hills. Climbing is not in me.