Hi,
It will cost you $12 per day to use one of the WDW reosrt gyms. But this is from allearsnet:
Pop Century Resort
Pop Century remains incomplete several years after the completion of the "Classic Years" (1950s-1990s), and running opportunities will no doubt change if and when the second half is completed. For now, though, a combination of the walking path that adjoins Hour Glass Lake and the outer perimeter of the parking area makes a nice near two-mile lap. Be very careful to avoid the high traffic entry drive. Starting at the front entrance (where DME drops off guests), run straight out the drive, turning left into the main parking area. Run along the outer perimeter to its end, joining up on the sidewalk that begins to the left. Run along Hour Glass Lake to the turn for "Generation Gap Bridge." Turn right, runnning the bridge round trip. Turn right, completing the run along the lake. At the end of the trail, follow the outer perimeter road, back to the entry drive.
I'm sure you can get more info at the front desk too.
Enjoy your trip!