A difficult one. I can see what you're wanting to do, but it's not really a great idea for your runners wellbeing.
Basically, the right shoe for each individual when running is different from the next person. This is down to the more obvious (width of feet) to the less obvious - biomechanics and running style. When you look on any running website listing shoes, you'll see them in groups such as "neutral", "stability", "cushioned" and "motion control". If a runner that has a running style that needs motion control shoes wears neutral ones instead, they're likely to get some quite serious injuries from it, particularly over 26 miles.
Your best bet may be to provide each runner with vouchers to go to their local running shop and get fitted for a pair that suits them - I'd have thought that local Up & Running or Sweatshop stores would only be too glad to do so if you contact them about it.