My commute is 49 miles round trip and I do it 3 or 4 times a week. Last Monday I took my bike with the steepest gearing and rode the return leg in a 25 mph headwind. There is a 4 1/2 mile long bridge in the middle of my ride and the winds were probably closer to 30-35 mph on it. I was in my lowest gear (42/24 for those who know about these things) and I was mashing at about 8 mph for 30 minutes to get across the bridge.
My
Ass
Was
Kicked!!!
I took a full week off the bike and realized I have been maybe suffering from a cumulative sort of exhaustion. I am 53 years old and in good health. My work day is full of physical activity, although I would say cycling is 75% of my exercise.
To the OP: like others have said, listen to your body but your mileage is manageable and you will certainly get used to it. Just do it in your own time and at your own pace.
And good on ya for being a commuter!