Whether we're doing something mental or physical, we feel that we're straining at it. That is the sense of effort. The greater it is, the less we want to work or exercise.
In the field of exercise science, there is a concept called 
Rating of Perceived Exertion (RPE). A high RPE means you feel more strongly that you are exercising. Put simply, it feels brutal. The higher the exertion you perceive, the sooner you give up. I want to broaden this into the more general idea of a sense of effort.
When you work, the less effort it feels like, the better. If something feels easy, it's easier to start and you keep at it longer. Burnout doesn't come either. Of course, some cases, like deliberate practice, need a fair amount of friction.1 But when it comes to sustainability alone, less is better. In fact, people rarely quit something because it's too easy and boring. Most quit because it's hard.
If lowering the sense of effort is a good thing, how do we do it? The first thing to try is thinking only about the near-short term. "Near-short term" (현단기, 現短期) is a term I made up.2 It means the next three to five minutes: not the present, but a short stretch pressed right up against it. You think about what you'd like to have accomplished once those three to five minutes are over.
With writing, for example, you focus only on writing the part that could be finished in exactly three minutes. You don't think about the whole piece. It may be similar to the Freewriting technique. You concentrate only on the part you can finish shortly and get it done somehow, first.
I picked this method up from working out. When you're running and still have a long way to go in time or distance, thinking about the destination actually drains your energy. The sense of effort goes up.3 At times like that, if you think, "Let's just make it to that point 200 to 300 meters ahead," you can do it. In other words, thinking only about the near-short term means working by focusing only on a small task that can be finished within a few minutes.
Focusing on the present, as mindfulness does, is actually poison. If you focus only on the present, you feel more pain. Mindfulness often involves body scans, but if you do a body scan during long work or intense exercise, you only feel more pain. That's why you think about the near-short term, which is neither the present nor the distant future.
This also connects to GTD. GTD says that if something can be done in two minutes, do it right away. A task that takes two minutes carries little burden, and putting it off is actually more of a hassle. In the same way, if you break work into pieces that can be finished in three to five minutes and pay attention only to that piece, it feels like you're only doing easy things, and the sense of effort drops.
Footnotes
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Another example where friction is needed is LLM Wiki and Germane Load. ↩
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It isn't an intuitive word, and it's not a proper term like "short-term" either, but I couldn't find anything better to call it. ↩
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Anyone who has done CrossFit will know this. Say the workout of the day calls for 100 burpees. If you think about doing 100, you're already out of energy. But if you think of it as 10 sets of 10, it becomes somewhat manageable. Since you're only counting to 10 each time, you reach 10 quickly. If you think this is just a trick of words, I suggest you try CrossFit. ↩