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My best mile and a half is 7.43 does anyone know how much I would need to shave off that time to be in a competitive race ?
It is not really a distance that is raced competitively. You will get more idea looking at times for 1500m or 3000m. Have a look on Power of 10 Web Site for the rankings of these 2 distances,
The world best for 3000 metres is around 7:20 so you would need to be able to run 20 odd seconds faster for an addtional 600 metres or so to be comepetitive.
The only timed one and half mile run I know is the army's standard PFT and that's a good time for it, I could never get below 9 min.