strange 5

very strange run. I can't think of having experienced anything similar, at least not in a very long time..

I got up extra early to run today, as I needed to be out on a bus to work early today. OK, no problem. Run started out well, as well as any day. A good pace - not particularly fast but on the faster side of average, felt strong, so far so good. In my fourth kilometer I suddenly noticed my legs feeling weak and tired and that I was slowing down, ok. I had been thinking of going to 6km, but with my slowing down and feeling tired I decided I'll just keep it to 5. After hitting the 4 mark and entering the 5th kilometer my legs felt even more tired out and almost rubbery. I could not even run any more. I walked the last 250 or meters to finish the 5km. very strange.