They should be able to reassure you, and usually insurance for a local authority will extend to volunteers leading walks by arrangement with the council, too. It is certain that if a member of staff takes member of the public into danger, it would be negligent not to warn them, and give the option of not going. Do not normally take guided walks along anything but the most sensible paths. In cases where someone would obviously not be suitable for the route, such as a frail old person, it is quite acceptable to tell that person that the walk would not be suitable for them. If they come, that is their choice.
Staff ratios One member of staff is the minimum requirement for a guided walk.
When events get popular and involve more than about 20 people, it is time to think about getting more people to help. Here is a table giving suggested numbers of staff and volunteers for public events. It has no legal force and is merely a suggested ratio based on experience. The difference between 'staff' and 'volunteers' will also depend on the sort of project you work for. Sometimes volunteers are the best leaders and know more about the site than paid staff.