system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why is it that we can walk hours without getting winded but 5 minutes of light running exhausts is?
gpt
Walking is a light enough load that we can fuel it entirely by the aerobic energy cycle and therefore, we never get a lactic acid build up, and can perform it practically indefinitely. 

Running uses energy faster than we can supply to our muscles through the aerobic cycle, so it becomes anaerobic. This generates lactic acid as a waste product, which is what causes the warm, burn-like sensation. 

After a while, the concentration gets so high that we can't keep making more without first giving the body a chance to get rid of the waste.

Long distance runners are at a cardiovascular fitness level so high that they can run at a decent speed while still remaining mostly in aerobic mode (because they can supply an enormous amount of oxygen to their muscles), therefore, lactic acid build up is very slow for them compared to the rest of us.