Introducing the Nike K9-Loper, the first running shoe designed for your best friend!
Something like this?
$80 looked kind of expensive, until I remembered that’s for a set of four rather than a pair. 
Interesting… They have a picture of a dog, and they have pictures of the shoes, but they don’t have any pictures of dogs wearing the shoes.
And no thread about animals in marathons is complete without a link to the definitive news story.
Sorry to take so long. Looking at the FEI rules (the governing body that oversees many of these long-distance endurance events), it appears that time during stops is counted on the clock, but I’m not 100% sure I’m reading it right and I’m not 100% sure that particular event is run under FEI jurisdiction. In any case, even if you added several half-hour breaks, the fastest horse would still have finished before the person.
Also, in endurance events, horses are not running. The average speed for the winner above was between 8 and 9 mph, which is a brisk trot. They may well take walk breaks (and in some races, they have to walk in places to get through rough terrain) and use an easy canter in other places, but they are not cantering or galloping the whole way, just as marathon runners are not in a dead run the whole way.
Horses are built for being on the go constantly - wild herds may move many miles each day between locations for foraging, resting, and drinking, in addition to shorter bursts of higher speeds as needed. They are not as fast as quadrupeds who are built for short bursts of speed, but they are very efficient running machines, and trotting for long stretches is hardly a chore once they are in shape.
I wouldn’t call it a cite, but this is an interesting article from Popular Mechanics.
When can a dog break into a sweat? 
I was running the Seawall around Stanley Park in Vancouver once, and a guy about 100 meters ahead of me had a very tiny dog (maybe 30 or so cm long), and the two of them kept ahead of me for about 40 minutes, running about my pace. I eventually caught up to them when they stopped for a break and I said to the guy I was impressed that a dog with such tiny legs could run that long without stopping. They guy said the dog had run several marathons with him and that once around Stanley Park was nothing.
horsetech, thanks for the feedback. Looks like we still don’t have the definitive answer. If you ever do stumble upon the definitive rules, please let us know.
Either way, I still think it’s astounding that a 50kg animal with 3 foot legs is even in the same league as a horse when it comes to distance running.
Hmmm. Perhaps that wasn’t the best expression to use.