Keeping up heart rate plan from trainer to the road?

Things are warming up here in the NYC area. It's enough for me to do a short ride, but it's still pretty chilly though.

I've spent the winter on the trainer, doing as much as 3 hours at a stretch. I've gotten used to it -- so much so that I didn't enjoy my first spring ride that much.

One of the things that took getting used to is all the new input. I'm training for a long-distance event and using a plan that calls for me to stay in Zone 2 for say, 3 hours, or Zone 2 for an hour with 10 minutes in Zone 4 at 15:00.

I'm finding it very difficult to hew to these kinds of directions on the road. I need to ride for at least an hour before I'm in an area where I can comfortably crank it up. Otherwise, I'm slowing down quite a bit for traffic, or to watch out for motorists pulling out of their driveways.

Do you guys have any suggestions for doing long rides in the suburbs at certain heart rate zones? Do you stay on the trainer, drive out to areas where there's fewer cars, or play stop-and-go with your heart rate, spiking it when you can and going slow when you can't -- essentially riding for distance.

I suppose I can go at low traffic times too, but it's still pretty cold! Another difference between the road and the trainer that I learned it -- as cold as it is sitting in my garage on the trainer, it's even chillier on the road. I really have to bundle up -- and come home soaked in cold sweat!

Thanks.