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Change it up. Speed workouts, distance workouts, hills, blah blah blah. Any running book or an issue of Runner's World can give you different ideas to incorporate into your training plan...
Unless youre training for the XC trail at NASP. I run pretty slow (11:20-ish). I ran the chip trail last week, 2 miles (4 laps) for 5 days straight. Today I timed myself for the 1.5 mi and got 11:00. And thats on top of heavy drinking Fri and Sat night and about 2 packs of Marlboro lights. A4s....kudos
I'm a Marine and 3 miles isn't that bad as long as you don't just get up one morning after not running for 6 months and go run 3 miles.
If I were you I would train with the goal of getting around a 20-21 min 3 MILE run time. That is my avg Marine pft score and when I ran the 1.5 mile Navy PRT at API it felt soooooooooooooooo easy cause I was used to running 3 miles. Ran it in 9 min.
Training for longer distance will ALWAYS make a shorter distance easier.
A good conditioning drill to throw in the mix is to hop on a bike at the gym and warm up for 5 minutes, then sprint for 20 seconds, then pedal slow for 40 seconds and repeat for 10-15 reps. (with a 3-5 min cooldown)
You don't lift the same way every day, so why do cardio the same way?
im training for the 3 mile, if ur training for the 1.5 i would do slow 8 min mile runs 3 times a week to like 3 or 4 miles. off set those days with ranger sprints to 1 - 1.25 miles. then on the final day of the week do ur 1.5 at full speed and drink a lot of water afterwards, try to keep hydrated. thats my advice... i went from a 22:45 3 mile to a 21:00 in 2 or 3 weeks... its works. its all about conditioning... good luck bro