Training plans are written as a best case scenario, but there is no way for you or your coach to know how your body & mind are going to feel on a particular day. No way to know how much of a toll all the other stresses of life have put on you. How much sleep you have been getting, the quality of your nutrition/hydration, any injuries, work, family, etc... All of these things can factor into your workout for that day.
Progression workouts are really quite simple, you start off at a pace you can handle and increase your intensity as the workout progresses. They are great for teaching athletes how to pace themselves at the beginning of a training session, so they will have enough energy to finish the session as fast or faster than they started. It is much better to dictate the pace you are going to move at rather than have the pace dictated to you because you went out too fast.
For those that train with power or heart rate, you can easily measure whether or not the intensity is increasing. While going on feel is great, the numbers don't lie and a good tool to use to compliment your training. Over time your feel will get better as your ability to gauge your perceived effort improves. This is important because you do not want to be a slave to the numbers and what happens on race day if technology fails you? You should have a very good idea what a particular effort feels like, and how long you can sustain it.
Remember that consistency in training is one of the keys to improvement. This coupled with a balance of stress plus recovery will enable an athlete to progress their training throughout the year. So try implementing a few "progression on feel" workouts during the coming weeks to help maximize the training that you do.
Train Smart... Race Fast!