I just got done a 20k run which was the farthest I've run in about a year. The last 3k of the run I slowed from around 5 min pace per K to well over 6 min pace. My legs felt dead and I could not lift them. My rationale was that even if I went very slow for the last few K at least I ran them, and next time I run 20k I will be able to run the last few faster. Is this actually true or should I have stopped when my body started to tie up? The last thing I wanna do is get injured.