First, see if you can run 3 miles in 27 minutes or under. then, start doing 3-4 days a week of conditioning.
2 days of this should consist of very high intensity training such as Burpee rounds or interval training, 1 day of a longer run like 3 miles with intermittant sprints (should not be easy), and then one long distance day like 5 miles. Work up to all of that. For the interval and burpee rounds start off with like 2 minutes rest in between rounds and each week knock off like 20 seconds of your rest, when you get down to 1 minute of rest, start increasing the rounds or intervals. For the distance runs , try and decrease your time a little each week.
When you can do a sprint workout of about 3 miles of intervals with 1 minute rest periods id say your ready to fight.
