Flogging a dead horse....badly

It's taper time, I should be celebrating. Loch Ness marathon, my first, is but 2 weeks away. I've done all the hard work (in as much as my make-it-up-as-you-go-along schedule dictates) and now it's time for that "rewarded" taper. Last Sunday I did a fantastic 24 miles. I struggled to force myself round, but after the half way point I felt great and was able to ramp it up to finish the 2nd half way quicker than the first. Good right? Even on those tired legs I managed to blast my way round the club session on Tuesday (3 x 8mins with 3mins recoveries) doing avg. 7.30 pace (that's fast for me by the way.....). Even on Wednesday morning I managed a decent paced 5 miler and on Thursday at the club managed to run ahead of my usual group in the pyramid session. So it's all good then? Hmm. Friday, I decided to have a well earned rest and on Saturday go out for a shorter long run - 15-18 miles, depending on how I felt. Saturday came but what a waste of time that turned out to be. The first 6 or so miles were okay. I hit the trails and explored the fields between Fordell Firs and Crossgates, heading towards Fordell Village and back into Crossgates. The road bit was a nightmare. My legs had turned to rubber and I struggled my way around the next 5 or so miles, the last of which I walked in a strop. I don't know what's going on. Even today I'm still knackered. I can't decide now whether I've done too much, too little or just enough.