I'm not a long jumper, but...

I went to track camp as a teenager and sat in on the long jump session. The one thing that I recall is that it is difficult to run full speed and then plant the jump leg because there is so much force on the knee/leg.
At some point, too much speed/force became a negative. They taught that it was better to either slow down a little or to "stomp" the step before the jump in an effort to dissipate some of the force before jumping.

I'll slightly disagree on the average jump suggestion. As noted below, the idea behind the event is who can jump the longest. They don't average times in the 100m or any other running event, the fastest in the finals wins. Its the same with shot put, etc... I believe. Think about the situation where someone jumps 21 feet on all six jumps compared to someone who jumps 23 feet, 22 1/2 feet twice and faults three times. The second person can jump two feet longer than the first person, but would still lose.