So, what does all this mean? When we look at the impact of an interval workout, we need to think of that impact multi-dimensionally. First of all, how does the way you approach the first interval impact each subsequent interval? Are you recovering easy enough between each interval? Secondly, how does the overall effort you put into a hard workout impact the next couple days of training? Thirdly, are you recovered enough heading into your next key workout so that you're getting out of it what you need to get out of it?
Training zones are zones for a reason -- because there is no pinpoint value that is "the best" in order to create load and force adaptation. For example, the default L4 zone is 90-105% of FTP (functional threshold power). If you do not train with power on the bike or you simply base your training off HR zones, then the L4 HR zone would be 84-92% of Max HR. If you were looking at an L4 workout on the bike of 2x20min, a common mistake would be to shoot for 100%+. After all, if 100% FTP is what you can conceivably hold for a 40k TT on the bike, then why wouldn't you push harder than that for 2x20min? The simple answer is that it's really frickin hard and you will be creating too much load for the workout. If you can hold 100%+ FTP for a 2x20 workout -- especially on the indoor trainer -- then you've miscalculated your FTP; it's too low. But, this aside, aiming for 90-95% is a perfectly acceptable range that creates plenty of load for this workout. Think about it this way. The point of L4 intervals is about teaching your body and mind to settle in, to cope with discomfort during the long haul. This is the goal of the workout. So, if instead you are pressing your limits and start the first 20min interval at 100%+ FTP, guess what's going to happen? Over the course if the first interval, your watts will drop slowly but surely. By the end of 20min, you'll be seeing stars and your legs will be torched. So, your second 20min interval will not even be close to that and you will probably be hard pressed to even hit 90% FTP. You will have short-circuited your motherboard and your body won't take kindly to more cracking of the whip. Instead, shoot for 90-93% FTP on the first interval. Think of it as the first third of a 40k TT and settle in. Finish the 20 minutes feeling like you challenged yourself but that you could have held that effort for another 5-10 minutes. Spin easy for 15 minutes (yes, 15 minutes; not 5 or 10), allowing your HR to drop well below 120bpm and holding your watts well below 55% FTP. Athletes press too hard during recovery periods when that recovery is meant to be very light. By pressing too hard, you negatively impact the subsequent interval(s). Now, for the second 20min interval in this example, start just below your average watts for the first interval. Give yourself a few minutes to settle in because the fatigue that is still in the legs will come back quickly. Let the body adjust, see how your HR is responding, before asking more of yourself. If you have played out your effort correctly, your second 20min interval will be anywhere from a few watts lower to a few watts higher than your first interval. If it is way higher, then you didn't press enough on the first interval; if it is way lower then you pressed to hard to begin with.
With this type of approach, come the next 1-2 days, you will feel better and less fatigued than if you pressed for the top of the training zone and cratered. Consequently, the workouts on those days will be of better quality (and by 'quality' I do not mean higher intensity; they will just be better and you will feel better). And you will have a better set-up heading in to your next key workout.
On the flip side of interval workouts are our long workouts, typically saved for the weekends. The challenge here is that athletes tend to press these as if they need to squeeze every bit of energy out of themselves in order for the long workouts to hold merit. We are so conditioned with the "no pain, no gain" mentality that if we settle in to a moderately aerobic effort for a long workout -- something that feels so easy compared to our interval workouts -- that we are indeed wasting our time. The purpose of the long workout is to teach the body to become more aerobically efficient. If we press the effort and squeeze everything we can out of the long workout, then the adaptation runs counter to the goal of the workout. If you are completing a 90min to 3hr run, for example, and the top of your L2 zone is, say, 145bpm, then it is critical to keep your HR below 145. Start the run in the 130-135 range and gradually increase your effort so in the final third your HR is 140+. There will be some natural cardiac drift, causing your HR to rise as you get fatigued and dehydration starts to occur, so if you start the long run at 140-145, then you will force yourself to either allow the HR to jack up into the 150s to hold the same pace, or you will slow down more and more in order to keep your HR below the 145bpm ceiling (in this example). Better to start the long run more relaxed, settle in, and pick things up toward the end -- which is how we want to race anyway, right?
Load and adaptation occurs over time, not just during the course of a singular workout or a singular interval within a given workout. My suggestion to all endurance athletes but to Masters athletes in particular is to practice restraint during your key workouts. Finish them thinking you could have done just a little more rather than wishing you had done less. By taking this approach, your day-to-day training will improve, you will get more out of all your key workouts and, over time, your progression will be greater than if you try to flog yourself every time you hit the bricks. The upshot is that you will feel better more consistently, you will gain confidence and, ultimately, you will race better as well.