I registered to sympathise. I did the Fleet half on the same day as Reading, which was my first ever race. Wanted to break 2 hours but missed my last long slow run due to a pulled muscle so lurked around the back early on and finished strong in 2hr6. The point being I actually didn't go that hard; I've felt more tired after training runs.
I then went on holiday the week after, drank moderately and ate, er, a bit more than normal but nothing spectacular. I' m not exactly a paragon of healthy eating anyway. Then I had the standard back-from-holiday blues for a further week, felt a bit ill as if I had a low-grade virus.
When I started again this week, it appeared I had forgotten how to run. Can't breathe properly, always seem to be going a stride too quickly and never 'gliding', getting fed up and bored very quickly. I have struggled with 3 miles which I used to barely even notice.
I think it's 2 things: 1) After a race, training isn't as much fun so it's easy to get demotivated and 2) the weather has changed. Before I got injured / tapered, I was training seriously in Jan/Feb, in temperatures usually between 2-8C whereas the last 2 days have been over 20C. I'm not acclimatised yet and I suspect the others on this thread who are struggling might be in the same boat.