Our bodies break down sugars (gluclose) into lactate. Our mitochondria use lactate as a fuel to create energy.  

Amount of lactate produced = flow of water. 

Amount of mitochondria = size of your bathtub drain. 

When we are at rest or doing easy exercise, our mitochondria can easily process all the lactate produced. As the run intensity increases (you open the faucet more) more lactate is produced until we reach a point called the Lactate Threshold. If we hold the flow steady at this point, we have one hour before the bathtub overflows. When we run faster than our lactate threshold, we fill up the bathtub quickly. That's when we can experience heavy legs and a desire to slow down. 

All running increases mitochondria density. So you can increase the size of the bathtub drain through lots of easy miles, but eventually you reach a max of how many miles you can run (but this is why elite athletes run 100 miles/week). There is something specific about training at lactate threshold pace that improves race paces even more than an improvement in VO2Max. This pace recruits mitochondria in the larger, more powerful muscle fibers. Lactate threshold pace also teaches us precise pacing and effort control, as well as developing mental tolerance for sustained discomfort. 

If we control the effort at our (unique to us) lactate threshold, we could in theory - with proper training and a good day - run for 60 minutes before failure.

Workout Adherence
We will never run more than 20 to 30 minutes at lactate threshold pace. More than that carries a high fatigue load. We are not trying to prove fitness during training, we are trying to build fitness. 

For the run/walkers.
Just as I challenge everyone who runs straight to master the run/walk techniques so they can deploy it from their toolbox, I challenge you run straight during the lactate threshold intervals. Warm up and cooldown using your run/walk intervals and then try to run straight. The work is short (relative to the entire run) and the pace is slower than your benchmark mile. If this is new, you don't have to be perfect. Attempting it will be progress.

P.S.
Lactate doesn't cause muscle soreness. That's a myth carried over from being able to measure something that science didn't yet understand. As soon as you stop the high intensity work the body clears the excess lactate.