Who are Zekenav, Roshav, Shoftav and Shotrav?
Malbim: They are, respectively, the Sanhedrin, the heads of the Shevatim, and the judges and enforcers in every city.
What was the purpose of his address?
Malbim: Many nations remained in the land that they did not dispossess. Either they will make a Bris and intermarry with them and become one nation, or they will fight with them until they dispossess them. Without this they cannot dwell in the land in serenity. Yehoshua feared that after he dies, Yisrael will fear to fight them, and choose the first way.
Why did Yehoshua say "I am old"?
Malbim: I cannot fight in war any more. In any case I did not conquer with my strength (rather, Hashem conquered).