Hey mememouse,
I ran with my dog in ET a couple of yrs back. Sure beats cycling! 99% of people cycle from what I have heard/seen. You will be the first, up front in line if you run, right behind the pace maker which is an added bonus.
I started training 6 months in advance of the run, more for my own sake than the dogs. We worked up to 15 km runs (remember you do get 15-20 min breaks between the legs) by doing 1 x 8km run break then 1 x 7km and so on. I found it much more difficult to run then break then run then break rather than do the whole 20kms at once. So make sure you practise intervals. The speed is slow (10km/hr), very comfortable jog. Make sure you take a timer with you so that you can measure your speed around a track to memorise what 10km/hr feels like (we jogged around 3km lake timing it). I could go on forever about training patterns/routines here but won't, the main thing to watch out for in your routine is to not get you or the dog injured. This means a good pilates and stretching plans for you and the dog, good diet and more rest days as training runs get longer etc. And definitely limiting the amount of running you do on hard surfaces like concrete to preferably 0%, this is where most injuries to dogs (pad injuries/stress and joint injuries) and handlers will happen (but I'm sure you already know a lot of this, having done marathons!).
Hope this helps.
:)