I Have Been Exercising Regularly, In Particular Bike Riding And

I have been exercising regularly, in particular bike riding and treadmill. I was regularly riding about 4.5 miles a day and walking another 1.75 miles. I did a three 12 to 13 mile rides and felt completely exhausted after the rides. Then my legs were aching so i took a break from riding for about 5 days and now it is difficult to make it the 4.5 miles. My breathing seems fine but my legs can't seem to take it. I am only 60 years old. I was struck by lightning in 1994 which burned my leg muscles pretty bad. Now this morning my left calf is hurting me like a pulled muscle but i din't do anything extra strenuous yesterday to hurt it.
Am I fighting an age issue or do yo think it is something else.