Effort represents an investment of a fixed resource, like calories. 
For this reason, running takes more effort than sitting. It takes more calories and strains muscles and joints. If you run non-stop, you will need to eat more to stay alive, and you will wear your muscles out.
However, effort as energy expenditure does not fully answer why we struggle to take action, as effortful tasks, such as playing tennis, is more fun than doing nothing.

Effort as Energy Expenditure

Self Improvement

Paying attention seems to be linked to effort, since deliberate control of attention take effort.
Focus is only hard if we're trying to focus. If our attention is held automatically, focus is not an effort.

Effort as Attention

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.

Effort as the Opposite of Habit

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.

Effort as Moving Upstream

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.

Different Kinds of Effort

Recently, I wrote a piece asking why taking action is hard. Why do we so often struggle to take our ideas and actually do something about them? It's tempting to just say people are lazy or unmotivated, but I think the question is a lot more complex and interesting than it first appears.