I won't get into the details of how carbohydrates transform to the energy our bodies need to function. 

From an exercise perspective what you need to know is that lactate is produced and is the dominant fuel source. As our exercise intensity increases the amount of lactate produced increases. Eventually, the production rate exceeds our ability to use (clear) it. 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. If we run at a higher intensity and exceed that threshold, we will feel a strong mental and physical sense of exhaustion. This lactate threshold is the point where our effort crosses from comfortably-hard to hard. 

For efforts faster than marathon pace we will never run more than 20 to 30 minutes of "quality" work. More than that carries a high fatigue load as well as higher risks for injury. 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 to use this lactate threshold mesocycle as an opportunity to run straight during the "work" 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 the benchmark mile you just completed. It doesn't have to be perfect. I believe you can do it. 

P.S. Lactic acid doesn't cause muscle soreness. That's a myth carried over from being able to measure something that science didn't yet understand.