I can sympathize. I've had that happen to me with running, too, going from always getting faster, setting PR's, winning my age group once, actually winning a women's masters once at a race, to now almost always coming in last. (I blame weight gain and MTBing taking all my attention for a couple years!)

Have you considered the possibility that you might have been over-training, or that, after your 159 mile ride, you needed a really good rest? It's easy to _say_ we understand periodization, where you go through phases of preparedness, building up to a peak, then coming back down, then building up again, but it's another thing to actually do it, because you feel like your fitness level should be constantly increasing, so it gets really hard to cut back or take days off or taper for a hard event or rest adequately after that event. (For example, I am doing what will be the hardest ride of my life next Sunday, a ride in Georgia over three mountain passes {the good riders are doing six!} for 50 miles, and I should be tapering this week, but oh no, I have planned rides of 60-80 miles for today and tomorrow because I can't stay off my bike and because I don't want to lose the long-ride-leads-to-weight-loss days.)

Or maybe there's something else going on that your doc hasn't found out yet, such as anemia, thyroid issues, who knows what. (I went to the doc once because I suspected I had a heart problem, {dangerous for the level of cave diving I was doing, but not really relevent to every-day living} and found out that not only did I have that, but a totally unsuspected lung defect, too! Nice!!)

Or maybe, at longer distances, are you getting adequate nutrition during the ride?

Just some things to think about, that you probably already thought of, but maybe not...There's nothing wrong with taking a break, though, and coming back refreshed.

Nanci