What is fallacious about it?

The claim seems to be that we often try even harder than is required to succeed. By trying too hard, we wear ourselves down, and might even cause us to fail in the process.

Therefore, putting effort beyond what is needed, by their definition, is excess and should be avoided.

Now I don't know if sometimes going a bit above what is needed can help in some ways, so I'm not saying it's true, but I don't see what is fallacious about it? The rationale seems to hold.

That's not the lexical definition of effort.

It's a stipulative definition that allows the author to reach a conclusion that's inherently provocative when read by people who are using the lexical definition.

> Therefore, putting effort beyond what is needed, by their definition, is excess and should be avoided.

By qualifying with "beyond what is needed" you've made it clear that you're using the lexical definition of "effort". I think that should drive home how absurd the author's definition of "effort" is. They've been careful not to make it a clearly circular definition (effort = effort beyond what is required) but they are awfully close.