It's not that many backs are not fast enough. Rather, the way defenses now play in the NFL, coupled with the speed of the OLBs and safeties, there is hardly ever a corner to turn.
I can't remember the last time I saw a RB turn the corner and go for a long TD. Even the long Willie Parker TDs looked to be a result of a quick inside cut, after a horizontal start to the plays.
Well, the only benefit to living in this forsaken territory is that I've got to see what a combination of excellent blocking and running can do.
I'll use AP's career day against the Bears as an example:
Most of his big runs came from exceptional blocking on off tackle-type runs.
At the second level, untouched, AP'd then make a hard cut back toward the middle of the field after the defender has committed to his angle. The defender, now badly out of position, gets burnt.
Another example would be when the Bears lined up in an Under with Briggs on WLB on the play. Peterson starts off like he's going for the B gap. Briggs reads and barrels in from the 9, just as Peterson makes a hard cut toward the sidelines. Peterson timed the cut to leave Briggs flat and breaks a big run to the outside.
My point is, inside or outside can work equally well if the RB has the time (and sense of timing) to get the defender out of position.
I use Peterson as an example because he does a great job (especially for a rook) AND he can do it without worrying about contact behind the LOS.