It's that time of year, people are riding or thinking about "base"..
For a long time, I thought "base" just meant riding slowly for a long time. But after reading Friel again, seems like it's more complicated than that. For one thing, it's not just about Endurance; it's also about Force and Speed Skills. (see here)
The other surprising thing I (re-)learned is that z2 is not just riding slow. So LSD is more like "Long Steady Distance" than "Long Slow Distance".
According to this, zone 2 is 81% to 89% of LTHR. Power zone 2 is 55% to 74% of FTP. At least by HR, it feels like harder than I've ever gone in Base. (then again maybe my LTHR is too high?)
And an interesting tidbit from the Training Bible is how do you know you're base is good or improving? When your heart rate drift is no more than 5%, you are in good shape. (source)
Anyway, some food for thought on those long steady rides this winter.