Yes, I know the feeling from the last km's of some marathons etc. No energy left in my system and the whole body is feeling so weak and you just have to rely on the will power. My late afternoon training was 18 km's around Lilla & Stora Delsjön + 8:an. I hadn't eaten any lunch, just some toast bread, so I more or less asked for trouble. The course around the lakes is very undulating, so it was hard work from start and already half way I was out of steam. Had to walk uphill and realized that my energy level was very low. Anyhow, I made it all the way according to my plan. Well, there aren't really any options when you are half way out there in the forest. Just keep moving forward. Got home and was a bit wobbly. I still think that these kind of low quality training sessions with empty fuel tank, are rather good in the longer perspective. I can recall some similar runs that temporarily destroyed my selfconfidence, that quite soon after gave me the opposite experience. Perhaps it's because I'm more cautious about charging my system with carbohydrates & energy, for a few weeks after when something like this happens.

A powerbar or energy-gel are always good to bring on longer runs, but today I only had water. I don't really have any respect for distances under 20 km's.