That's in the clause...
"a player in an offside position is moving towards the ball with the intention of playing the ball and is fouled before playing or attempting to play the ball, or challenging an opponent for the ball, the foul is penalised as it has occurred before the offside offence"
If just making a run is, by definition, "challenging for the ball" then what possible scenario do you think this clause refers to exactly? Why has it been included?
It clearly doesn't mean that just making a run is automatically "challenging for the ball" for a couple of reasons...
1. The whole clause would be redundant;
2. The clause clearly distinguishes between moving towards the ball and challenging for the ball.