Easy peasy? Convert to an array first of course. Or, if some of those columns are non-numeric, then convert only the indicated columns to an array.
But it is pretty easy to generate that list of columns.
T = rand(450,42);
M = mean(T(:,[2:5]' + (0:5:35)),1);
So the list of column indices is:
ind = reshape([2:5]' + (0:5:35),1,);