Agree w/Larry ... the fatigue you're describing is common and not necessarily a sign of any kind of lack of physical fitness.
It does get somewhat easier with time.
The right pair of shoes is critical. What the "right pair" is varies from person to person. Shoes that are great for walking are not necessarily great for standing.
Take advantage of opportunities to sit, and don't feel guilty about it. Can you sit while you start an IV, explain discharge instructions, etc. ... sneak in some time off your feet whenever you can.
The particular environment of your unit will dictate things like lunch (ha!), breaks and pit stops to use the bathroom & have something to eat or drink.
Good luck to you!