Ok. To clear this up.
I might sound a bit Malmo-ish, but it running is very simple. Find a correct balance betwenn volume & intensity.
When you really want to rock I will tell you something:
It is NOT a question of 50 MPW singles or doubles. When you are fine with 50 MPW on singles I advise you to do MORE miles AND to run doubles.
When you are able to run 7 miles per day, just add 2 or 3 morning runs of 3 miles in week 1. Then increase the number of doubles till you reach 4 - 6. I like 5 doubles per week. Monday - Friday. So I have just one dayly run on the weekends. I you have tuble doing your normal 7 milers, just cut it short. For recovery AND training purpose it's better to do 5 + 5 than 1 x 7. Believe me.
Try to work up to a schedule like this:
Mon: 5 + 5
Tue: 5 + workout
Wed: 5 + 5
Thu: 5 + workout
Fri: 5 + 5
Sat: longer run (could be 9 - 10 miles)
Sun: 5
Or like this:
Mon: 5
Tue: 5 + 5
Wed: 5 + workout
Thu: 8 - 10 (longish)
Fri: 5 + 5
Sat: workout + 5
Sun: longish run (9 - 10)
Very simple. And better than 50 with just singles. When you are able to do this consistently - try to run more. Training is about steady progression.
And one more time: It is NOT about e.g. 100 MPW in singles vs. 100 MPW in doubles. It is about 100 MPW in singles vs. e.g. 120 MPW in doubles. With doubles you are able to run faster and more volume. And in the end that's all training is about.
Pikachu