How can I apply an excel formula to every seventh line?

I have a spreadsheet that I use to keep tracking of my running progress. I’d like to start looking at some of the data by week rather than by day. The data are organized by columns, so I’d like to be able to, for instance, sum every seven rows (for seven days) and look at total miles or total running time per week.

I’ve tried to work out a formula using the mod function (=if(mod(row(), 7)=0, sum(C1:C7)), but I can’t quite get it to work.

What am I doing wrong, and is there an easier way to do this?