Fatigued

Feeling a bit fatigued this week. Not real sure if it's from the race effort on Monday, the long (busy) weekend, or maybe just the change in weather, but I am dragging! Got in an easy 6.5 miler on Tuesday evening. Couldn't run Tues/Weds/Thurs mornings because of meetings, but even still I felt tired on Wednesday night so I switched my track workout from Weds to Thurs. Did another easy day of 5.25 miles on Weds before heading to the track this evening.
On the agenda tonight was essentially the same track workout as last week but with one additional rep: 20min WU, 5 x (1000m @ 3:26 pace, 3 min (500m recovery jog)), 20 min WD. I felt like crap on the warmup but managed to run alright on the track w/the following splits:

3:27, 3:26, 3:26, 3:28, 3:31

By the end I was cooked. Although my overall mileage for the week is only at 35mi in 4 days (my lowest mileage by Thurs in weeks), more than a third of it (13.1mi) has been quality mileage at 6min pace or less, so maybe that's what is tiring me out this week. Anyone else in Denver dragging this week as well?

Ahh well, easy mileage day tomorrow followed by long run Saturday. Can't wait for the end of the week!!