If you are not used to riding 20 miles at a shot, twice a day, then you will do this one or two times and give up because it is a lot of time and energy that you're using every day.

My advice is to build up to it. Drive the car for 10-12 miles and bike the rest. It's much easier to do 8-10 miles each way. Then slowly add more bike miles to your commute as you drive shorter and shorter distances.