Scubagirl,
to do the average speed calculation you want use the following =(distance)/(time * 24) and make sure that the cell format is set to "General" (excel will try to outsmart itself by setting the cell to a "Time" format).
The above presumes that your "time" cell is already in a hh:mm:ss time format.
Cheers,
Jamie
P.S. in other places where I have tried to deal with time in excel, I have found sometimes that I needed to divide a value by 24 to get it to produce the time correctly.