Be careful with that thought process. I started out the same way and wish I hadn't. Customers who are happy with the results will ask you back again, then change their minds when you tell them the new price. If it is a friend, then you can get away with it, but anyone else is risky.
It does take experience to learn how to estimate your time and set a price. You will likely not make as much per hour at first, but as you speed up your $/hr will increase. Discounts aren't always a bad thing, especially when business is slow. Make sure the customer knows what the regular price would be and why you are giving a discount. (Multiple cars, friends & family, referrals, etc.) Quality work is the best advertising you can do, so the extra time is not always a loss. When I spend extra time on a detail, I simply view it as an advertising cost. Usually much more effective than any other form of advertising I could spend my $ on.