I guess for me, the thing was that they were just completely different rides. The 400k wasn't just a 300k with another 100k tagged on.

More specifically, most of my 200k rides and the first two 300k rides I did were basically solo rides. I don't mind riding solo in one sense, but it does bother me after a while, and I sort of dreaded doing anything longer because I figured it was just going to be a matter of riding around by myself in the dark all night.

Well, on my first (and only) 400k, sure enough, the group(s) started off at rocket pace, and I had to pick my own pace. I got out almost to the 200k control when I met the second group coming back (it was an out-and-back route). Then when I got back to the 300k control, having ridden that first 300k by myself, I pulled up at the control and there were a dozen bikes lined up in front of the store. They had waited about 15 minutes on me. I can't begin to tell you how wonderful that sight was! So I got to finish that ride with some of the other riders, which was so neat. And when I was done, I think I felt better than I had when I finished my first two 300k's. I discovered that an easy 400k (meaning, minimal wind problems in my case) was easier than a hard 300k (but not faster).

Since then, I've done two 600k's and another 300k and some shorter rides, and have been mainly hanging with the same group, and it has been a blast. The rides vary, but riding with other people makes it so much better.

You might consider riding in an area with more riders for a changeup if you basically ride solo, or if you're having wind and mountain problems, ride someplace less windy or mountainous for a changeup.

Something else I notice on longer rides, not just 400k's, but shorter ones- nutrition plays a big part. I had some rides last summer where I just felt so exhausted, and I think the issue was just not getting enough calories in because I was drinking so much. Stay on top of the calories, try Perpetuum or something while you ride, and see if that helps.