I find that 60% of FTP is a pretty safe average wattage. "Safe" meaning I usually don't have trouble eating enough at such an intensity level and fatigue is more of a function of how long I have been awake rather than fatigue due to the riding itself.

FWIW, 60% of FTP is about 2 W/kg for me. (Well, used to be, I have gotten fatter since I sorted out these numbers....) I should note that these numbers were for up to 300K distances and 15 hours.

These kinds of benchmarks can be useful once a few long rides have been done to 'calibrate' them to different people's abilities / fitness levels, etc. I'd caution anyone against using the figures I just threw out there (60% of FTP and 2W/kg) - other folks may see higher (or lower*) numbers.

* Especially if they are talking about longer rides than 300K.