This workout - tempo in the morning, 300's in afternoon - was done at Stanford under Vin and on the Farm Team (for those of you old enough to remember). The workout serves two purposes.
1. The problem with Tempo Runs is most top runners do them too fast. I often remember out on Sawyer Camp trail knocking out 5 min miles off the back. An afternoon session puts the tempo in perspective and helps the runner keep things under control. Another way to do this, is run with no mile markers and go off effort. However this often became an XC type race as guys who were feeling good pushed the pace and took the pack with them.
On a side note, there may be less races on the schedules but many times a "tempo" is an unofficial race on the calendar.
2. Great way to get in some more mileage with the extra recovery. After your tempo you cool down... But coming back in the afternoon you have another 2-3 mile warm-up, strides, 8 x 300 with a 100 jog, and a 2-3 mile cool down. That is another 8 miles for the day.
Some people throw in some fast stuff following the tempo...we did that too on the Farm. But this way your legs are better prepared for some good turnover work. It is a short 60 min or less workout that teaches you to run fast while tired from a morning session.