When I was training for 3mile run, I was given a work out plan by my command and it read something like:
Monday - run .5miles as fast as you can, rest 2min and repeat until 3miles
Tuesday - 80% 4min run, 60% 2min, repeat for 3miles.
Wednesday - regular 3mile run at your best pace.
Thursday - 300 yard sprint followed by 1min rest x6.
Friday - forgot
Saturday - we would do 40lb rack sack on the beach for 2 hours.
All these runs were after a regular workout.
My time decreased by 5min after 1 1/2 month.