If the weather is nice, just simply take a walk. Nothing is better that just taking a walk. Tread mills and other exercise equipment is great for working different muscle groups, and when the weather is bad, but nothing beats the action of what your training for.

Walk around you neighborhood, go to a park, where I live there are rolling hills and plenty of trails and sidewalks where I live that have inclines and declines.

I read in a "How to" through hiker handbook where a through hiker was quoted as saying, "I did all my conditioning on the trail", meaning that he really didn't do any preconditioning, he just hiked at his own pace and as the miles went by his pace improved.

If you have or think you may have some health concerns, make sure to confide with your doctor.