
There is a mini schnauzer who does canix (cross country running with dogs) and that is a usually a 5 - 6K run, it seems to do ok :)
Also I dont know if you are planning to get both pups at the same time but it isnt generally advisable as two are more like triple the trouble as they bond with eachother and are more difficult to train. You may be better off getting one and then in a year or so when that one is fully trained get another. There arent many (if any) good breeders who will sell you two pups from the same litter. Your best bet is to google for the breed club and find out about any litters, you should also be able to find out about relevant health tests etc to make sure you get yourself a healthy running partner :)