I've spent a fair bit of time on this blog talking about this topic already, particularly around the idea of heart-rate based training. A couple of very relevant posts are: Heart-rate Training Part 1 and Heart-rate Training Part 2.
The key aspect of the heart and lungs for ultra running is developing a massive aerobic engine. It's not often in an event longer than the marathon distance that you are consistently running in an anaerobic state - your body simply can't sustain that effort for an extended period as you'll burn up your stored glycogen (your fuel source at higher intensities) and cannot possibly fully replace it whilst running at high-effort output. You'll either force down supplementary carbs and your guts will protest, or you'll skimp on additional fuel and you'll bonk. Neither is ideal.
We hear all the time about the magestical fat-burning (I know - magestical is not a word - but I watched "The Hunt for the Wilderpeople" yesterday) and turning your body into a "fat-burning machine". In theory your body has enough fat stores to fuel a run for a damn long time, certainly longer than any standard ultra-distance running event. There's a need for a small amount of sugar to kick-start the fat-burning process, but otherwise if exercise intensity is "low enough" and your body well-trained to burning fat, theoretically you have enough energy to keep on running without requiring supplementary glycogen. "Low enough" is simply below the limit at which your body starts to dig into your stored glycogen and/or require carbohydrate intake to fuel itself.
The beauty therefore of developing a strong and efficient aerobic system is that your body is able to utilise fat as its primary fuel source, which facilitates running for an extended period of time without flopping to the ground in a sugar-deprived state of bonk-ed-ness.
So how do we build this huge aerobic engine?
This is where I suggest you go up and read my blog posts linked to at the top of this page.
I'm a massive believer in the Maffetone approach to developing aerobic capacity, which is, very simply, running up to the limit of your aerobic zone as often as possible, and for as long as possible. Using this approach, you train the body to run faster and faster for a given HR, meaning you're training your body to run faster for a given output within your aerobic zone. Same effort, same primary fuel source, but faster running. Couple this with a near-endless supply of fuel (in the form of fat) and you have a very sound base from which to go and tackle an ultra. Sounds good, right?
Depending on where an athlete is in their aerobic development, I might also suggest the inclusion of one or two higher intensity sessions in a training week (sessions where you are above your aerobic threshold) for the purposes of fine-tuning your performance level - this is the 80-20 approach I talk about in the linked blogs (I'm not going into the full details of why we do this - email me if you want to know more).
But - and here's the kicker - you could run purely aerobic all of the time and build a sensational ultra-running machine. The inclusion of higher-intensity running has its benefits, but for lots of athletes the additional stress induced by such sessions isn't worth it. Focus on your aerobic development - build that huge, efficient aerobic capacity - and make the intense sessions the exception, not the rule.
One thing worth nothing with all of this is that focussing on remaining aerobic during racing, or training for that matter, can be an arse. Walking up a hill early in an ultra and watching other runners fly by isn't particularly morale boosting, but you need to make peace with this - I've become a habitual hill hiker in races, and I'm most happy to let people huff and puff past me, knowing I get to be this smug arsehole in a few hours time...
Lastly, focus on rest and recovery. Damn it - make it your priority. Stress is stress, and if you aren't moderating your training based on what else you have going on in your life, you'll be doing yourself a disservice, and can't possibly expect your body to optimally adapt to the training you're doing.
"So what do we do now?"
...huff, puff, huff...
"Wait, wait, wait. No, no. Maybe we don't need to run."
"Huh? Oh yeah, let's just fast walk.