As maintenance is a physical activity, there are parts of the job that are just that; a part of the job. Some properties I worked required multiple stairwell climbs, walking long distances, pushing/pulling/carrying heavy objects, reaching and holding awkward positions and other types of physical activities. To be honest that's one of the things I loved about being on site. (Sometimes, I kind of miss it now.)
To make it more endurable, invest in good footwear. I used to go through inserts in my boots regularly, and knew it was time to replace them when my knees would hurt at the end of the day.
Plan your trips up the stairs and make use of boxes, cart's and/or dolly's whenever possible to decrease the amount of trips. (My favorite is still a 5 gallon bucket)
Some basic stretching before the day begins can help as well... You just might be able to cancel the gym membership!