The theory on doubling the distance is to build endurance; however, she should not be running longer distances daily. I've had to do this with my son because he has a slow mile time. The theory is that if you train your body to go 3 or 4 miles regularly, then a 1.5 mile sprint is a walk in the park because you've increased your endurance, muscle strength, and lung capacity to withstand higher cardio levels.
The downside is, as yozongu said, it will wear you down faster. Distance should only be done 2 or 3 days a week and run sprints or intervals on the other days. You definitely need down days if you're running distance.
One good method is to pick a route with several up/down hills. One day a week walk the up hills and run the downs, this will increase speed and leg power. The other two days walk the down hills and run the ups, this will increase endurance and lung capacity. The next week alternate. On the flat surfaces maintain a steady jog pace to keep your heartrate up but not exhaust yourself.
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