I agree with the above, head palying tricks, I had a similar problem before last weekend, I felt really bad, I had completed a 35 mile run end of May and relaxed back, possibly too early, I put it down to taper problems (only did minimal miles in June) so I did a couple of short runs and quite a lot of biking this week to put my legs back in exercise mode and it feels really good. I think for me, tapering can have the effect of letting the aches and pains take hold but thankfully it seems it's temporary