Effort could be seen as the opposite of something we do automatically. Effort then is what happens when we try to override an automatic pattern.

You can look at effort in the context when you need to make a choice that involves internal conflict. Part of you wants to eat more, while the other part thinks you shouldn't.
Internal conflict requires you to move "uphill" or resist the easy flow.

Taking action is hard because it requires effort. Some things feel easy, while others feel difficult. This is a reason we don't always act on our plans.

But, maybe the problem is that we have combined distinct issues under the same label of "effort": physical fatigue, attention control, automaticity, self-control. If we could separate them, we might be able to ease the effort required to take action.