Why am I stressing out?

On Monday I started my new job. It's a legit career. Great pay & benefits. But I'm constantly brooding about how I now have to work 40 hours a week. I mean, I'm doing it. I did it this week. But why do I have to be such a scrub, mentally? Lol. "Oh no, you have to do moderate physical labor for half of the day, most days. Maybe stay overtime and get paid even more." I'm a whimp.