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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.