CERC's Wednesday Workouts are designed to progress you through different paces and distances, with the goal of maximizing efficiency at race pace. If you look closely at the workout schedule, you can see the progression as we build up to longer and longer distances at goal pace in conjunction with shorter efforts at faster paces and longer efforts at slower paces.
Those longer efforts are meant to build up your aerobic endurance and your capacity to hold a pace for longer. The shorter efforts are there to increase the pace you can hold! And the race pace efforts get you used to running at that race effort.
How do you determine your paces? The most important thing is to use your CURRENT fitness, not where you hope to be in 12 weeks. Some veterans runners may be able to run what they could run in a 10k, for example, on THAT day, given weather, surface, recovery status, etc. Don't worry if you have no idea though! This is a learned skill. If you aren't sure where to start, an online race time calculator can be useful. I personally like the VDOT calculator for it's simplicity. You put in your most recent race result, click calculate, and head to the third tab of results to get your equivalent paces across a range of distances.
That said, it is important to do things by feel. It is ALWAYS better to overcook than undercook a workout. Why? You have to run again tomorrow, or the next day! Training for endurance events is always about the whole body of work, not about any one workout. If you go as hard as you can on every day, you're going to risk injury or burnout. If you get injured, well, it's going to be hard to be consistent in your training.