What Causes Weakness In Leg Muscles?

I am 48hrs old, fit and healthy. I walk my dog 30mins in the morning, 2hrs before tea and 30 minutes before bedtime. I am out walking for 3hrs with him during the day at the weekend. The muscles in my legs feel weak and when I walk up hill I feel puffed out? I am tired in my self lately even though I am getting7-8 hours sleep. Any ideas?