I do 100 miles of commuting a week, so about 20mi a day round trip. When i started I'd feel dead by the end of the week. In the beginning I'd take friday off and have a good ride saturday of 40-50 mi. I got used to it though and went 5 days commuting with 50-65 weekends and now I'm fine with it. Sometimes I take it easier on friday just to enjoy and wind down, finish the week off with a smile, etc.
I'm switching things up now and will be commuting 5 days a week but will add weight room work and then MTB on the weekends.
Your fitness will change and you'll get better at it all.