I was able to extend the length of my walks only slightly in the first several years I was sick. From time to time, I tried extending my walks by a few minutes a day, but most of these experiments produced increased symptoms, leading me to return to my previous level. 

After two years, I was walking about a half hour a day, not much different from the 20 minutes or so when I was first sick and very much less than the 15 miles or more a day I hiked before CFS. 

I was finally able to expand my exercise in a significant way when I incorporated rests into my walks. I discovered how I could apply rest to exercise when I went walking one day with another CFS patient, a person who had a more severe case than mine. 

She stopped frequently, about once every five minutes. I noticed that I wasn’t at all tired at the end of our hour-long walk, and asked myself whether I might be able to extend the length of my walks by incorporating rest into them.
 

So I tried an experiment. The next day I walked for 20 minutes, then sat on a bench for a 20 minutes, then walked some more. It worked! I was able to walk significantly farther than before by including a long rest in the middle. 

This was a big psychological breakthrough, as it opened the possibility of longer walks without an increase in symptoms. I was able to extend gradually the distance I could walk, as long as I alternated periods of walking with equally long periods of rest.
 
Over a period of several years, I returned to doing hikes as long as those I took before getting CFS.
 

(Adapted from RecoveryFromCFS.org)