The exact "work you need to do" is pretty much given to you when you do a degree. If you work hard on it and complete it all then it is hard not to succeed.
When doing a start-up or in the "real world" you can easily fill up time with work which is not actually directing you towards your ultimate goal(s).
When you're in school, it often doesn't seem like the work is given to you. I remember knowing this, and I remember when it switched over for me in different subjects or different contexts. I'm not sure that start up world or "real world" is any different, there are just new (more difficult) contexts which can start out seeming mysterious but can become routine (often in retrospect).
When doing a start-up or in the "real world" you can easily fill up time with work which is not actually directing you towards your ultimate goal(s).