i commuted by bmx for a few years. 44/16 or 36/13, 5 miles each way. i loved riding and had no life outside of work. just go home and ride some more. it was a blast for the most part, except the days i injured myself on the way to work and had to stick it out through the work day and ride home.
a 20" has no problem keeping pace with a regular non-spandex rider riding a "normal" bike at a comfortable pace.