Hi there and welcome.
Always glad to see more people interested in going a little bit further with their dogs.
Keep an eye on the NSDSC website for further details of the Tayside event, we're still waiting on some of the finer details to be confirmed.
As for your questions -
building distance is quite simple -

- you just need to run further.........
I'd suggest you set yourself a target of the level (distance/speed/whatever) you'd like your dogs to be performing at. With Mals, you're not likely going to be able to influence speed for them unduly, what you should be looking for is consistency. Can they run the last mile at more or less the same pace they do the second and third? The more they run, the fitter and stronger they will become and the more you, as the driver, will need to focus on controlling that initial burst and extending it further into the run.
You can also play with their heads a little - as you push the distance up, and they get used to the increase, it's always fun to throw in a short, easy run. It helps their confidence and also keeps them happy.
You'll want to pay close attention to feet, and shoulders. Watch for pad wear and nail length. Your trail surface will dictate how much care you will need to take.
Obviously temperature plays a part too.
You've mentioned the Tayside event, so that would be what you would focus on. Ideally, you want your dogs to have run the same format as any event you enter - so you'll want your dogs to know what it's like to run 3 times in 2 days - and over whatever distance it is. The longest leg on the Triple is likely to be around 9 miles. The night stage will be much shorter.
I hope some of that is of use to you.