Carbonfiberboy 

They're probably going quite hard. I don't know of anyone who climbs in zone 2, even on all day events. The best aerobic conditioning I've ever found is simply riding with a group that's faster than you. That's way better than doing intervals on your own. Getting dropped is very good. Keep riding with that group. I got dropped every ride for about a year, that's once a week, but I held on longer every ride. We rode once a week, all year, rain or shine. Eventually I wound up leading that group. I'd tell the newbies to "hang onto their wheel until the blood spurts from your eyeballs." A group ride will teach you everything you need to know about cadence, pacing, nutrition, and clothing. Cadence: always spin about the same cadence as the rider whose wheel you're on.

That said, that's just once a week. A 3-5 hour group ride is all the high end conditioning you'll need. The rest of the week, ride Z2. You probably won't be able to profitably do anything else, but do put in some miles. Strength training in the gym twice a week is a good idea, too. I usually did a ride the same day, but only an hour or less, before the gym. If you don't do the group ride every week, you will need to do some intervals to make up for it. This Z2 only thing is taken from pros and elites who can put in 15-20 hours/week, but they are also doing group rides or runs or skis of varying intensity.

When I started my year's training in October, I'd put in a lot of Z2 hours, working up to 2 hours on the rollers 5 days/week for about a month, which was very helpful. That was plenty of low end work, but that's a lot of riding, many people won't be able to even do that. By now, I'd usually be doing some sort of midweek intervals too, maybe low cadence Z3 hill climbs or long periods of max cadence work on the rollers. Only 4 months to July. I kept up doing this routine every year into my 70s.