Workout the day after donating blood. The difference was noticeable (at least I choose to blame my diminished energy on yesterday's blood donation!).
5x1 mile at lactate threshold effort with 1 minute rests. 8x200m at mile effort with 200m recoveries. I sweat a lot and was ready to be done. 11.42, including warm up and cooldown, which together were almost 4 miles. (11.42 in 1:23:00)