I use the commute as training, as I don't have lots of time at weekends with family stuff. Let's say this, as a day to day commuter, when I go on club runs and more noticeably, any sportives, those of us who ride every day are up there with the guys that train and race. You will kick the back side off the weekend warriors.
Only the really serious guys are faster, and they are faster because they race as well as ride every day.
Commuting is excellent for training the body to deal with the daily stress in muscles. All that stop start with traffic is a real killer at first, but once your body adapts to dealing with the lactic acid, you will make great improvement.
I see commuting as an ideal fitness base that fits in with your routine, without impinging on family time. Essential for those with families.
I commute between 20 and 30 miles a day, usually only 10-14 miles in one go, but I can be on the bike 4 times some days with site visits. None of it big distances, but I don't have any problems jumping on the bike and riding 100 miles despite hardly ever doing those sort of distances.
Regular commuters are generally some of the fittest cyclists (racers excluded).