If you are doing 600 miles a month, you will do fine. Don't over-think the miles and do what you know how to do. Just keep hydrated and fueled. I normally ride close to and very often more than 800 miles a month; last month was 747, this month is at 430. Before my riding buddy required shoulder surgery, we were doing 1 to 2 66 mile rides a week and at least an 88 or century ride once a month.