It happend a few times the past few weeks. I did two lovely long rides, nice 3 to 5 hours in the saddle with lots of bird watching. Uploaded the ride to Strava - obviously gave it a good name otherwise I won't get kudos - and then laid back on the couch watching the cycling on TV. Especially during the periods that I upload these kinds of rides, I regularly get asked what I'm actually doing. Is this useful? And whether I had left my form at home?
The answer is simple, I'm using this to prepare myself for the goals that will hopefully come. The explanation of why training slowly (or rather at low heart rate) is good is often more difficult to explain to novice cyclists. If you want to know exactly, feel free to Google for a while. There are plenty of sites that explain in detail terms such as aerobic, anaerobic, VO2MAX, FTP, SST, all kinds of different zones (where Z1 for some is different from Z1 for others, let alone how that relates to D1), various thresholds and mitochondria.
We use these terms regularly ourselves, such as recently in the article about how to make the wind your friend. But the novice cyclist often gets confused because of the terms and slightly different explanations. If you are an experienced cyclist (or can Google well), then you don't need to read this article. If you really want to know everything, then Google all these terms further or contact a CycloWorld editor Karen Oud, who's a' professional cycling and triathlon trainer. But if the science of training is something new, then the way I explain it to my (cycling) friends might also help you to better understand the idea and thus become a better cyclist.
Every cyclist knows that at the end of the ride they need to have something left in the fuel tank. You don't want to be the one who has to drop at the end of each ride. What you want to avoid - just like a car - is ending up with an empty tank. However, the cyclist is not an "old-fashioned" car with a type of engine, but a hybrid car with an afterburner.
When you accelerate gently in a hybrid car, you are only using the battery. As long as you don't accelerate too fast and don't go above a certain limit, you can keep running wonderfully on that battery. The fat in your body and oxygen can be compared to the battery in a car, with the difference that it never actually runs out in your body (as long as you keep breathing).
If you ride faster or accelerate too hard, the car automatically switches to gasoline. Then you can go faster, but the tank has much less capacity than the battery. Your muscles and carbohydrates are like the gasoline in your car. If you exert yourself more, your body switches from fat and oxygen, to muscle and carbohydrates. You go faster, but your tank gets empty quickly. You can refuel (eat and drink), but you can never replenish the tank as quickly as you draw it empty. Somewhere along the way - no matter how well you eat - the tank gets empty.
Finally, on the bike you also have a kind of afterburner. That's when you're going all-out in a sprint of on uphill. The acid that you then feel in your legs, also works as fuel for a while. The comparison with the afterburner in pimped cars also applies here. The afterburner only works very hard for a short time and if you use it too much you flood the engine and it does nothing at all. This also applies to the cyclist, once really soured you won't be able to move forward. You can use this afterburner in two ways; as small additions to your gasoline or use it up completely at once. In both cases, this fuel will eventually be depleted between 5 seconds and 5 minutes and your muscles will be too acidified (drowned) to move forward. Just as with a car, you can fix the "drowning" by leaving the engine alone for a while. Pedaling into acidification is fine, as long as you give your body the time to clean up the acid again.
The good news here is that unlike the car, you can train your body to use any kind of fuel longer and better. But then you have to train that energy system. If you train a lot on your battery (low aerobic), then over time you will be able to run longer and harder on your battery and your body will only switch to the gasoline engine later. This is because a cyclist's battery is actually a whole bunch of little batteries (mitochondria) that become more in number, larger and more efficient through these workouts. By specifically training to repeat longer blocks at a high heart rate, you can also improve the quality of your gasoline engine (high aerobic). Just like you can train the moment your body switches to the afterburner (the tipping point). Riding just below your tipping point is what we call the Sweet Spot, this is where you can use the powerful fuels the longest without blowing up the engine. This too can be trained by Sweet Spot Training (SST). You can also train how long you can use the afterburner as a mix with the gasoline (anaerobic). And even your afterburner can be trained by repetitions of very short and very fierce onsets (anaerobic ATP). But you only really train these parts if you train very specifically for one part. Out the door, get tired and done is nice, but in terms of training no more than maintaining the old car with 1 engine. In the end you will not be much better. Variation is therefore important.
Why is this important? Most of us want to enjoy the trips we make in the first place. Therefore, train mostly on the battery. Low heart rate, high cadence! You'll have more time and energy to enjoy the surroundings, talk to your mates. And - importantly - you'll always have enough in the tank to get over the climb with the group and turn on the afterburner to secretly push your wheel as the first of your group across the finish line.