I have always followed the rule of thumb that one should maximize their mileage in singles before jumping to two a days. 70 miles a week is the number that is usually thrown out there as a dividing line. However, I know that Joe Rubio is strong proponent of doubles, particularly on your \"hard\" days.
According to Mr. Rubio\'s Middle Distance Guide:
The second and more important reason we have 1500 runners do AM runs particularly before any harder afternoon workouts such as track work or tempo runs is because of an interesting study conducted in Italy a number of years back. It showed that the human body produces elevated levels of human growth hormone (HG) in the bloodstream during the 4-6 hours following a run of 30-40 minutes. It is theorized that an AM or lunch runs doesn’t necessarily loosen you up for the afternoon run as much inject the body with naturally occurring hormones that allow the afternoon workout to be performed with less effort. This is the reason that most top 1500 runners will run an easy 20-40 minutes without fail a few hours before a big meet. They know this easy run will make them feel better for a hard effort performed a few hours later.
Running that two-a-day may in fact make you more capable of hitting the quality sessions hard. However, it must be also noted that Joe says,
A recovery run is typically defined as 20-45 minutes of slow and easy jogging. The idea is to move the crap out of your legs, not to get in an extra 4 mile tempo run. The pace should be no faster than 65% of your 5k pace at any point in this run (usually in the low to mid 60% range is what we want). If you allow 6+ hours between sessions, doing one of these easy recovery runs in the AM and another in the PM is fine and within the definition of a recovery day if you’d like to maintain double days within the schedule.
Eventhough I am currently at 60 miles a week, I may try to incorporate 2 morning 30 minute shake out runs to see how my body responds.