Riding in the drops is really key for maintaining speed, particularly into the wind. I use them quite a lot, but chiefly just for short sprints or for the duration of a pull in a pace line - 5-10 minutes tops, usually - but I also find it hard to stay in the drops for long periods of time. I don't have any problems with my back and I think my core is pretty strong, but being in the drops allows one to derive power from different muscles - this is part of the advantage actually - most notably the glutes, and I just don't have the fitness in those muscles like I do in my quads, etc. And so my legs fatigue rather quickly if I try to spend long intervals in that position.

People will tell you to do different strengthening exercises or stretches, but I think the real key is just spending more time down there. 9 miles straight is probably not the right approach though. I would suggest (to myself and to you) making sure to do a mile or so in the drops every 10 miles, even on rides when you are not hammering. Just like easy recovery rides are important for the muscle memory of normal spinning, riding in the drops position and having a different assemblage of muscles firing is good training. That's what I've been working on recently, so that when i really need the power and the aero benefit of getting low, I will have it.

I know not everybody here is a group rider, but one thing that I learned for pulling in a pace line is that as soon as I find myself in front, I get down in the drops. Pulling successfully is not usually about a great increase in power output, but if you are out in the wind, it helps a lot to be as aero as possible, and that means getting low. I commonly see people out in front and way up high, with hands on the hoods and elbows locked. Unless they are really strong, they are probably burning up way too much energy that way.