This is what it is to be serious about something. Suddenly, you are doing the work instead of going for a pleasant stroll. There's nothing wrong with the pleasant stroll in itself, but some purposes require a different attitude, what I am calling seriousness and the work.

Likewise business and entrepreneurship - I read lots and lots on technology, financing, market research, marketing, etc. But now that I'm really nailing down one aspect for my next business, it becomes almost strenuous to work on that.

It is strenuous. That's what makes it work -- in both readings of that sentence. If someone is unaccustomed to doing the work, it can look like nothing but a burden, or a wall of Ugh.

A few examples.

One of my colleagues served as head of department for a few years, and in that capacity he would have to see students who were failing in their studies and give them a talking to. He developed a routine for this. He would see them in batches of a dozen at a time, and he would ask them, "How many hours of studying did you do last week?" Already to say "hours", plural, was optimistic. Some of them weren't doing anything at all but turning up for lectures, and some not even that. So then he would ask if any of them did something like playing in a band that got gigs, or in a competitive football team. In a group of that size there would usually be one or two, and he would ask how much practice they put in. And the answer would be something like, "we've a training session on Tuesdays, and a practice game on Thursdays, and then league games on the weekends--" they were, in other words, doing the work on these things, because they were serious. And that, he would tell them, is what they needed to be doing in their studies.

I've had an amateur interest in artificial languages for a long time, and then a few years ago I was invited to write the article on artificial languages for a linguistics encyclopedia. And there were all these things that I sort of generally knew about, but now I had to seriously read up on and think about and get critiqued by contacts in the conlang community, in order to write a professionally informed exposition in an academic book. So, after getting over my initial reaction of "What? Who? Me?" I started on the work.

My brother is a professional classical musician, as was my father, and they've both said that the biggest difference between amateur musicians and professionals is not that one does it for love and the other money, but the seriousness that the professional brings to it that most amateurs don't (with rare and wonderful exceptions). Of course, the professional must have a certain level of seriousness, because it's the food on his plate and the roof over his head, but to be an outstanding musician takes even more than that, or he's just "digging his daily trench of Mozart" (as James Galway once put it in an interview), or doing the job instead of the work (Seth Godin).

Another concept in the area of being serious about something is being a geek about it (as long as it's not just intensely focussed work on trivia).

Or cargo cult planes versus the real thing.