For your run training for the 5k you are thinking about... running 2x a day will actually hurt your run time, imo. I just got done with one, as you know.
Alwyn Cosgrove's website has the best information about how I trained for my run/race, and I knocked off 1.5 minutes from my best 5k cross country time AND I'm four years older now.. plus I used my bands for my legs in the gym, and my prototype for cardio/run training.
Run ONLY 2x week, but do it this way: break up the 5k training into one mile repeats, and 3/4 mile repeats. Do each interval (1 mile or 3/4 mile) as hard as you can.. rest about 1/2 the time it took you to do it.. then do that hard interval again... rest, etc. DO THE 3 MILES EACH WORKOUT - NOT six miles, or eight, etc. Either three intervals (1 mile each) or four intervals (3/4 mile intervals). Each week try to decrease the rest between intervals to build up your endurance... NOT by running for longer time periods or more miles. That only reinforces running slowly, not as fast as you're able for the distance you're trying to run or race.
This will give you better quality training sessions, burn fat, and you won't be able to run the other five days a week, due to being tired... and you'll have more time for LIFE.