What exactly take takes up a lot of time and effort starting a business

I have heard (and believe) that starting a new company takes huge amounts of time and effort. Having never done it myself I don't know where this time and effort is normally expended. Say for a tech start-up where the software has already written before launch, what should I expect to have to spend most of my time doing?

Forming a company in most modern economies is a fairly trivial process.

Starting a business takes significantly longer. Things we found take a long time:

1. Product development. Initial build is not so bad, but refinement to 1.0 (elimination of bugs and usability issues) can take a long time. The initial build is the '80' of the '80/20' rule, but you need to address a lot of the '20' to have a product.
2. Product marketing. Getting your website, sales collateral, and of course SEO takes a lot of effort - it's the public face of your product so it has to be really, really good.
3. Support. Not so much handling calls, but getting documentation done, setting up FAQs, tutorials, walk-throughs, etc takes a lot longer than you would think, and it always suffers to the sales/marketing side for attention. In early stage not all of it is important straight away, but you need a certain bare minimum.
4. Business processes. Small business doesn't need lots of processes, but you still need to get the fundamentals in place: payment processing, bank accounts, paypal accounts, license generation, legal things (eg license terms), support processes, email accounts, some form of CRM, some way to deal with accounting.

This is akin to asking "What exactly takes up a lot of time and effort fixing a car?" Having only worked for a company you don't get to see everything that goes on to keep the business running (under the hood).

I recently made the switch from "individual contributor" to entrepreneur. I have to understand bookkeeping, sales, applicable (and peculiar) laws, marketing, financing, et al in addition to the product that I am actually selling.

It's not one particular thing. It's everything.

My experience is in business software and for me, what takes interminable amount of time is connecting with the market.
Yes the software is written, it may have even been tested with one-two customers, but that is just the beginning of the story. You will need:

1. The website: 10 pages at least, for bis software, explaining ins and outs, offering trial etc etc. If you are a one-man band - 2-4 weeks before you are not ashamed to show it to the public.
2. Traffic to the site: a PPC campaign is the fastest way to get traffic - 2 months before you get to the right quality
3. Website update: once you get some visitors, need to fine-tune your site so that they stay on the site and progress towards free trial or contact you - another month
4. First enquiries: it is unlikely the people want carbon copies of what you did for your test customers. More likely, you have a good idea but they want a variation. how long: any amount of time.

Why most businesses fail, especially in software? They don't have the persistence, don't stay around long enough to properly connect with the market and iron out all the problems. If I am to single out the most important factor for business success I would say it is the power to stay and execute.