I am trying to describe a working state in a period of time.
A few years ago, I was running my own business. At that time, I was working 15 hours a day for around 10 months.
... I had been working 18 hour long days over a period of around 10 month.
... I had worked 18 hours a day for about 10 months.
Are those all grammatically correct? Which one is better?