I do a commute a little longer than your 20 miler and wear a variety of things. Today I rode in with Dockers under which I had a pair of Pearl Izumi liner shorts. These are like road bike shorts but are thinner, have some padding and are made to wear under other pants. For the return trip in 75F weather today I just put some shorts over the liner shorts and put the Dockers in my pannier. Last week I rode in and back home with some 3/4 length knickers that also have some padding. I also have a Brooks B17. When I was younger I could ride without the padded shorts, started needing them at about 55.