When I used to weight lift a lot, one of the things that I learned which was really important was when to back off during training, or just not train completely. You basically had to be able to distinguish between when you were just a little tired but you still could work out, and when you really shouldn't work out like when you're injured or sick or burnt out, etc. You also have to figure out when your brain is just being lazy and you can workout, even if you don't feel like it.
Backing off during training can be a hard thing. We're all hard charging athletes and we always want to give it our all for every workout. We follow what our coaches give us and if we don't follow their plan exactly, then we get an anxiety attack and feel like we'll never finish our goal race. If we miss workouts, we feel like we're not gonna make it.
The reality is that you can have what I call "backing off" at the macro and micro level and still be fine for the race. Macro level is when you get sick or injured, and you have to take off multiple days (maybe weeks) to get better before you can go back to training. Micro level is when you either have to skip one workout due to some factor that will resolve itself within a day, like not being recovered enough from the day before, or extra soreness, or low energy levels. Whether you can have a great race depends on so many factors and not just if you miss a workout or two, or even for a week or more. Generally, backing off at the micro level, ie. taking a day off, skipping a workout, reducing pace/watts, etc, doesn't have much effect if it happens temporarily. Backing off at the macro level for too long, ie. being sick for a month, taking weeks to heal a pulled muscle, being lazy on workouts for many weeks, etc., will definitely affect performance.
So don't worry about it so much! Back off because you should and don't stress about it. But do pay attention when you back off for a long period of time.
How do you tell when to back off? It took me a while to figure out how to tell when I should back off and how much. Here are some things I learned:
1. If I'm sick, then of course I should not train.
1a. But figuring out when I can start training again after being sick can be tough. When I finally feel physically a bit more energetic, I usually start with recovery workouts to get the body readjusted to training again, before leaping into full bore training. I try to keep workouts short, like 15-30 minutes, and not overstress my system until I'm 100% back.
2. I have tried to fine tune my sensitivity to my physical condition. This is intuition, and knowing how your body responds to stress and how fast it recovers, and a sensitivity to the condition of your muscles and overall system. Only experience can tell you what your body needs on that day. So keep alert and a log if it helps, and get to know your body as much as possible.
3. I find that the five key things to keep track of are your heart, lungs, muscles, energy level, and brain. Warning signs are when my heart rate is a bit high, or my lungs feel stretched still from an intense workout the day before (I usually feel what can be described as a "cool" sensation in the middle of my chest), or my muscles are sore or tight. Or sometimes my brain just doesn't seem to have the same willpower as it may have on another day. Or my energy level is low and I feel tired. These all tell me that I need to be more mindful and potentially may need to adjust my workout to fit.
4. The easiest days are when I approach my workout and feel mentally fresh and my body feels energetic. The other easiest days are when my body is overly sore and I feel mentally and physically tired: an obvious sign that I should take a day off. The toughest are days when I am right about to workout and am on the edge of feeling not all that energetic and maybe I'm not fully mentally psyched for working out. But through experience, I know that often when I start the workout, my energy comes back to me when I start.
On days like this, I usually don't worry about being lazy - it's not really in my nature nowadays and I always look forward to working out. But I don't know exactly what I can accomplish during the workout, and am not sure whether I can push the limits or just do a recovery workout, or somewhere in between. So I usually am prepared to adjust my workout midstream to what I can handle at that moment.
I find that it is during the warmup that I can figure out most of the time what awaits me in the rest of the workout. As I warm up, I approach my goal workout speeds (swim or run) and/or wattages (for bike workouts) and see how my body responds. If I am feeling like my exertion level is too high and unsustainable, then I know I'll have to lower paces or wattages during the main set or else I will flame out before the workout ends. If I feel good, then I will be OK for the goal paces/watts.
Another potential factor: sometimes if I approach a workout feeling a bit tired, I can bring my workout up to goal paces/watts simply by having a longer warmup. By getting my blood flowing slowly with more time, I am able to get my system moving and potentially still hit my goal paces/watts.
5. If I make it past the warmup, then I may adjust again during the workout if something is not right. This can be a perceived exertion that is rising too fast for me to make it through the workout. It can also be a sapping of willpower that could evaporate if I am powering through some really fast paces or watts and trying to hold them for long intervals.
6. One macro level adjustment I have made was my recovery in general. By doing so many intense weekend rides doing Old La Honda and Kings Mountain climbs, along with a 2-3 hour run, I have found that I actually need another 2-3 days of recovery. Just taking one day off is not enough; I will do recovery workouts for 2 days after and then I'm fine on the 3rd day for a normal pace/watts workout. I know this because I have tried to sustain paces/watts after one day and my exertion and heart rate leap in the first few intervals.
Instead of fighting this, I just merely added it into my workout regime and it has not affected my race times at all; I'm still getting PR times race after race. It's almost unintuitive that you could rest more but still yet race faster! I chalk it up to my age, my fitness level, and what I need to do to race faster.
The important thing to note is that we're all different as humans. We all come from different fitness backgrounds and levels, and that coupled with our age and genetics means we can train a certain way. We need to develop a sensitivity for what our individual bodies need, and not stress about how others are training. I am a big believer in individualized training and I think this is where a lot of generic plans and group training can harm people. Developing an awareness of how our bodies work, and coupling that with a good coach (who won't train you like you're in the army and/or shame you into doing senseless workouts), will work wonders for your race performance.
So take a day off if you really need it or back off on the paces and watts - don't stress about it and you'll still race fine come race day.