Some passengers use the gym which is great but I prefer the fresh air.
Sun Princess top deck is only a short walk and exposed to the heat. As we are cruising the north of Australia it is hot so I found an alternative for my walks which is deck 7. On this deck you can walk the entire length if the ship so it is a good walk around and is sheltered from the sun. 3 times around is 1 mile apparently.
I have been having afternoon walks on sea days for at least an hour and have found it very relaxing and have not had to worry about sunburn.
Another great tip on a cruise is to use the stairs instead of elevators. I must admit that going down many flights of stairs is more appealing than going up!! I have to confess that I don't follow this tip enough!!
Another small mercy for us has been the location of our stateroom. We are on the very back of the ship and our balcony overlooks the wake of the ship.
This means that to get anywhere we have to walk further so I estimate that we walk a few kilometres each day just going back and forth for our stateroom from the various venues on the ship.
Anyway I hope this information is helpful for those of you who may travel on Sun Princess........