Roughing the Passer enforcement

[RazorRef]

Rule book says penalty enforcement for roughing the passer is assessed from the "end of the last run".

Case book has a situation where A1 is roughed after his pass to A2 who catches then fumbles. The ball is recovered by A3 ahead of where A2 lost possession. (I don't have the case book on me right now to get exact yardage). The penalty in that situation would be assessed from where A2 lost the ball since that was the end of the last run.

My question is, what happens if A3 picks up the fumble and runs another 20 yards before being tackled. Which run is the "last run". Where would enforce the roughing the passer penalty?

Thanks in advance.

RazorRef