It is really tough. It feels like there is an elephant on your back, you feel flat, your legs feel weak, you're really tired, you have a hard time breathing and it's painful. At this altitude everything is so much more painful. If you overextend yourself just a little bit, especially when you're in a technical section and you have to put out a lot of effort just to get over something, the moment you're over it you're in trouble and it takes a very long time to recover.