It doesn't really matter, just don't beat up on yourself. Keep it fun. I do a mix of zones year 'round. It's not necessary to keep it down, in fact it works better to do some hills, just don't knock yourself out on them. A minimal base is usually 1000 miles, more is better. I try to get in a couple thousand before I do anything serious. So I'll start base training in October and do my first intervals in February. This year it's too late to start a structured plan unless you live in Oz. So just ride. do what you want to do, and see how it comes out.
Whatever effort level allows you to ride consistently for longer periods e.g. 3+ hrs. If you ride 3 hrs and you can't or don't feel like riding long the next day then you went too hard. Generally, a comfortable pace that doesn't leave you wiped out after 3-4 hrs.