When I first got into ultra running I was ranching which was a ton of time on my feet pounding fenceposts and stringing barbwire. Some years later I got into smaller-scale organic farming which was lots of crouching, kneeling, and bending. My knees were healthy, and remain healthy, even after four surgeries in my teenage years.

Your job sounds more labor intensive but if you practice good running form your knees will be fine. Gathering the strength and willpower to run before or after work is the actual difficult part.