finding the centroid between two points

Hi, I have a very large data set that contains two sets of Lat/long. One set of lat/long is for the start point and the other for the end point of a particular event. What I need to do is find the centroid of each start and end point for each row (event) found within the data set, there are over 26,000 rows found in each of the 6 sheet for this particular project.

Thanks,
Jane