Instead of doing 9 in the morning and 3 at lunch, I did 12 in the morning. I forgot my Garmin, so I'll have to guess that it was ~9 min/mi pace. I have kind of a weird feeling in the groin area (stop that snickering!), so I didn't want to do a workout. I decided to do 12 because frankly doubles are a pain.
I could really tell the difference after the distance was over 9 miles. I ended up "pleasantly tired" as Lydiard would say. 12 miles was cake in the spring when I was doing 12 every Tue, 14 w/ last 5 at MP on Th, and 20+ on Sunday.