Runners have a tendency to track their workout times. This can be both misleading and dangerous if you don't figure in the effect of weather, a trap both newbies and experienced runners can fall into. Consider long runs.
Last week, I went down to the local canal trail for a long run. I ran 16 miles and comfortably averaged about 7:00 minute per mile pace. In fact, my first few miles were a bit easier and I had no problem running the last few in the upper 6's. It was mid 50's F, clear, and low humidity. A downright refreshing and pleasant run.
This morning, my training partner and I ran down to this same trail. Now, over the course of one week of normal training there's no way that you're going to lose or gain considerable fitness (short of accidents and illness). We ran 12 miles, averaging about 7:30 pace on the trail. We were drenched and exhausted on the final approach to his house (and a welcome dip in the pool). We ran 3/4ths the distance at a considerably slower pace and finished anything but "refreshed". How did this happen? Simple. This morning we ran in mid to upper 70 degree weather with extremely high humidity (as of this writing around noon, the dew point is 74 F). And to top it off, the sun started to burn through the mist about halfway in for added sauna-like effect. So we've got warmth and extremely inefficient evaporative cooling due to the high relative humidity.
If your body can't cool itself, you will suffer and be forced to slow down. It doesn't mean that you're out of shape, it means that you're a human being. In fact, we expected the run to be difficult, and so set off at the afore-mentioned easier pace, but still, the effects are unavoidable in spite of our experience and caution. I'm only glad we didn't opt for 14 or 16 again.