Some days you can double. Some days you can single.
Listen to your body. Some days you might do 4 miles in the
morning and 6 in the afternoon. Other days you could run 10 miles straight.
I wouldn't bother with a run if its less than 4 miles.
Heres how I like to set up a 60-70 mpw base training schedule. Flexibility in training is important!
Day 1 - 8-12 miles easy @ 65-70% heart rate (single or double), strides (100-200 meters)
Day 2 - 8-10 mile steady state run @ 80-85% heart rate (10-12 miles total)
Day 3 - 8-12 miles easy (single or double)
Day 4 - 8-12 miles easy (single or double) / hill sprints (10-12 seconds in length)
Day 5 - 3-5 mile tempo @ 85-88% heart rate consisting of either intervals or a straight tempo (7-9 miles total)
Day 6 - Off or 8-12 miles easy (single or double)
Day 7 - 14-16 mile long run @ 70-75% heart rate