During the walk, keep an eye out for anyone who is struggling. Stop to allow people to catch up if the group starts getting strung out. Consider having extra rest and water stops if necessary. Check the head count and the pace regularly. Check regularly that you can see your back marker, particularly at junctions.
Make sure that gates are closed after the group has been through and that dogs are kept under control particularly near animals. Try to keep a watch at stiles and other obstructions to make sure they are safely negotiated. Remind the group to keep to the sides on roads, particularly if you know they are busy. Walkers should face oncoming traffic and proceed in single file. Warning should be given about care when crossing roads.
Don't be afraid to ask for help especially in unforeseen circumstances.