I have also heard both sides of the argument many times before, I can speak to what has worked for me in the last 5/6 months. In the past I have struggled with getting my average pace and race day pace up, I think this was as a result of attributing the wrong objective to my long runs. I used to view the long run as an opportunity to increase my weekly mileage, I now view the long run as an opportunity to recover from the week's mileage and test my mental ability to continue running on tired legs. Generally my long run is 10% - 15% slower than my average pace. However to supplement the above, all my runs during the week are at a high tempo (including hill repeat days). I have noticed a dramatic improvement in my average pace (that pace that you can run without checking your watch or without feeling like your legs are holding you back). In the last 5 months I have improved my average pace from approx 6min/ km for a 21km to just slightly above 5min / km average for the same distance. My 2c.
