sed -e :a -e 's/^.\{1,77\}$/ & /;ta' # method 1
sed -e :a -e 's/^.\{1,77\}$/ &/;ta' -e 's/\( *\)\1/\1/' # method 2
NOTE:
In method 1, spaces at the beginning of the line are significant, and trailing spaces are appended at the end of the line.
In method 2, spaces at the beginning of the line are discarded in centering the line, and no trailing spaces appear at the end of lines.