exhaustion with outdoor activity

Devin, Hi..my question is this: I try to do some type of yard work or walking outside 3 times a week to keep myself moving.. Walking I last 15 minutes doing almost a half a mile or 30 minutes outside watering my raised garden beds or weeding at ground level. No matter what the temp except for extreme heat my right leg starts to weigh 100 pounds and I get short of breath.Why? Is this because of the physical exertion on my body being outside? I always have projects in the house where I stand as much as I can due to being afraid I will lose the loss of my legs sooner if I don't do this. I dont get as fatigued or have legs problems. Can you shed some light on this? Thanks , Barb