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There is a training plan that I was following when I thought cutting down the number of days would help with the ongoing issues I was having (before I realized what I really needed to fix). I had read an article in Runners World which stated that you can still perform/pr while training on a 3 day running cycle. Basically you are doing a tempo/speed/long run (cross train on off days). There is a book you can purchase (of course) at the time I was able to find the plans online, but now it looks like they have updated the original links that I used. Here is info in case you are interested:
The Furman Institute of Running and Scientific Training (FIRST):http://www2.furman.edu/sites/LiveWell/GetMoving/Pages/FurmanInstituteofRunningScientificTraining(FIRST).aspxThe FIRST - Training Programs Main Page (from here you can link to the program):http://www2.furman.edu/sites/first/Pages/FirstTrainingPrograms.aspx