Can I make a suggestion, with regard to a route. If possible, design a sort of 'clover leaf' route with points passing close to the start, so that people who can manage less distance can finish at a point near the beginning, while the folk who can do more can go on and do another 'loop'...
Of course, you'll need to keep the pace down for those who are less speedy. If you have enough people, they can choose a faster or slower pace, and you can have a shorter and longer route to a cafe point or whatever...