Try to arrive at the start in good time so that people are not waiting for you this also allows you time to check everyone against your list, even if everyone on your list has arrived do not start before the stated starting time as people do turn up who have not posted to say they are coming.
Before you start you could give people a brief description of the walk and any points of interest to look out for.
Try to set the pace of the walk to match the slowest member so that the group does not get too spread out, inevitably the group will spread but you do not want this to be too great as you need to make sure everyone sees which direction you have taken, also it can be a little demoralising for the person at the back if they are struggling to keep up.
If the turn out on the day is quite large you may want to ask one of the experienced members to act as a back marker to make sure no one is left behind.
At the lunch stop make sure you allow people enough time to enjoy their lunch, although this may be dictated by the weather on a wet day if there is no shelter.
At the end of the walk make sure everyone who set out has returned and if you are going to a pub or tea shop afterwards make sure everyone knows where you are going and how to get there in case they want to come.