A question about the time to walk 1 kilometre?

I will be doing my silver DofE soon and I am looking for some advice on how long it will take my group to walk 1 km. There are seven of us in the group, with a range of fitness levels. There are also 4 boys, and 3 girls. We will be carrying on average a 65L rucksack that will be full. Ive noticed that Naismith's rule dictates that a km is roughly 20 mins, but i was wondering what you guys on here with some experience possibly would have to say. Thanks very much