Sufficient.

10 miles each way, 26 minutes~summer fixed gear and load all the way up to 50 minutes (or more!) on the knobbied/studded MTB with load mid-winter and storming.

Yeup. Sufficient.

It's all fun to me.

I can't help but ride hard, must be in my genes. I wish I knew what a recovery ride was. I may intend to ride easy but most the time a good tune comes up and off I go. That plus I have numerous hills on my commute and steep ones at that.

Keeps me pretty damn fit though!

My commuting pace depends on the weather/length of commute its either a all out sprint or a fast pace ride. I use a HT MTB and today it was an fast pace ride with my speed staying in the 17-24mph range. An all out sprint for me is usually when i go to school and is 2 miles in 3 minutes with 2.1 knobbies( i havent done it with slicks yet).

I think perhaps your Lycra shorts are a tad to small

I wish I knew how fast I could ride sustained on flat ground, as my commute is all hills. Right now, my speed varies from 6-40 mph, depending on whether I'm ascending or descending. It takes me about 30 minutes to ride the 9 miles to school, so I guess I average about 18 mph.

Mine is an amusing mix of long stretches of wide open mups that I can haul arse on and numerous streets with quite a few stop lights and stop signs. One particular 4 mile stretch has 11 stop lights.
So, a sprint in those sections.
It really is what I make of it depending upon the purpose of the ride.

pretty slow these days, I'm out of shape so I've been taking it easy.

tomorrow will be even slower since I'm fighting off a cold.