Activities Report - Script Runs Too Long and Can't get the rest of my activities

I am trying to get all of the activities on People and Companies using custom reports. The instructions say that “Due to the script execution time limit of 6 mins from Google, this process may need multiple runs.” It tells you to check the last row on the second sheet. If it matches the last row on the third sheet you know it is complete. My last rows are not matching. But when I try to run the script again, I receive this error: “A sheet with the name "User Activities on Companies" already exists. Please enter another name.” How can I get it to run all of my activities?