I have 3 different routes I can take, 3.5 miles, 4.5 miles, and 5.5 miles. If I'm late for work I take the short route, if it's nice out and I'm not late, I take the long route. I take the middle length route in the winter because I can be sure the roads will be cleared on that route. It's nice to have options.
I ride at about a 15-16 mph average pace, not too hard, not too easy. I'd say it's in my upper green zone bordering into yellow. I definitely sweat on warmer days and get out of breath here and there. But I try not to push it hard like I would on a road ride. On the way home, I definitely push it hard, almost time trial like. Doesn't matter if I get sweaty then, I'm going home!
Depending on how long your commute is, you don't need to really change your clothes when you get to work. Short routes aren't long enough to get body odor or stinky clothes.
If you're getting tired legs, take a look at either your diet or how hard you're riding. Maybe back off on your effort and just take it slightly easy. After all, you're riding a bike to work, it's not a race, no need to go overboard and red line.
But really, the key is to just ride more if you want to get into better shape and not be tired all the time.