You will need to try walking the route before the day of the walk, possibly even several times to make sure you are confident of the route. Try to find someone to go with you on the recce, prefereably an experienced walk leader, who can also go on the actual walk (a potential back marker who will know the route). On the recce you should:
You may do your original recce months in advance of the walk, to check that it is suitable for the group before submitting it for inclusion in the programme. If so, you should repeat the recce very near the date of the walk just in case of unforeseen problems. You should also confirm any arrangements made, such as parking or pub lunches.