I did it years ago on a quilt and left an extra inch so I could fold under 1/2 inch and then fold the 1/2 inch over the quilt. The only problem I had was with the fabric wanting to get a wrinkle after folding it over along the long edge. You should be alright on a table runner since the distance is shorter. Use lots of pins or maybe a little washable glue so it doesn't travel.