What training plan to prepare for starting HR zone training

no point traing to do a training plan just crack on you'll get more benefit in the long run your basically signing up to 20 weeks of training using your method those first 8 weeks could be junk miles with no learning or benefit.  British cycling do training plans on the website and is well worth joining if your not already a member.  As others have said Trainer road is also very good and they offer virtual power which is a far better method of training i've never found training with HR that reliable and would favour RPE (Rated Perceived Excertion) over it but every one is different.  

Do you have a heart rate monitor?  Your fears about not being able to control your aerobic zones are completely unfounded, if anything using a HR monitor makes it easier to stay within them as without it is easy to stray above your target.

You will need to set your zones though and rather than trying to find your max HR I would suggest finding your Lactic Threshold HR and setting your zones from there. Here is how to do it https://sporttracks.mobi/blog/determine-your-lactate-threshold

Once you have these set then simply ride in Zone 2 initially to get an aerobic base before starting to buold on this.

I question the validity of your plan...

Whether you start your planned HR programme now or in 8 weeks time, you should be using a HRM straight away. 

HR is a great training metric, and ironically, the importance of HR in determining effectiveness of certain training efforts is increasing again after a few years of poo pooing by the power meter brigade. 

Personally, things I'd encourage you to initially do is identify your maximum heart rate. From there you can work out your zones, and use this 8 week time to get used to riding in those zones. 

Your fitness will not dictate your ability to stay in a zone, this is achieved by learning how to moderate your pace. 

For cadence, is there anything in particular you'd be wanting to achieve?

Two cadence based workouts I undertake (but with power and heart rate to gauge effort), are as follows...

1. ramps. After a nice warm up, complete 3 sets where, starting from a cadence 10 rpm below your normal average, you increase cadence by 5 rpm per minute until such a time that you can no long hold form. this completes one set... 

Give yourself a minute easy spinning and then repeat a further two times... followed by a nice warm down. 

You need to keep the effort relatively light throughout, so lower resistance as the cadence goes up.

2. Variable Cadence Tempo - again, a nice 10min warm up, then complete 30-60mins at tempo intensity. When riding at tempo (on HR), you need to build the effort slowly, so spend 5-10mins slowing increasing the pace until such a time you are just inside your tempo hear rate range. Don't change the intensity of effort throughout the remainder of the effort. You'll need to achieve this by monitoring speed as your HR will naturally drift upwards over the course of the effort. 

If HR does go above your tempo range, you are going too hard and need to ease off slightly. 

Throughout the effort, you should look to maintain speed, but change cadence every four minutes. This is super easy on resistance rollers (I just change gear) but may need some thought on a turbo depending on how the resistance is provided. 

Some 4mins should be at your normal cadence, but vary cadence so that you spend time significantly under and significnatly over your normal cadence. 

This session is a good tempo workout, but also help you develop pacing and also pedalling flexibility. 

Oh man, I've droned on there... 

As mtmTomo says, if you've got a HR monitor I would just jump straight into that. It doesn't mean you can't prepare for a more intense training period by first starting with a few weeks relatively easy to get your aerobic capabilities up, it just means you have something to measure your effort by. 

HR is a great indicator of your effort so I would suggest getting on the trainer and warming up gently for 5-10 minutes then finding a gear/cadence that is enough to get your breathing up a bit but where you're not working too hard (if you know your zones then I would say stick with high Z2 HR to start with), and tick along and target keeping your HR relatively stable. Don't worry if you have to slow down, you'll get the power endurance back. Working at this kind of level is also great for practicing some skills again e.g. spend 5 mins at a slightly higher than normal cadence followed by 5 mins at a slightly lower cadence and 5 mins at your 'natural cadence'. That kind of thing also keeps it a bit more interesting.

Long term if you have a trainer getting some estimate of power should be quite helpful for training - if nothing else it will give you encouragement as you see your output rise for the same effort level and heartrate. I've never bothered to invest in one for riding outdoors but since getting a turbo trainer in September I've found it pretty helpful. As mentioned 'virtual power' via a speed sensor is a cheap way to get a good estimate although ultimately it's mainly the effort (and HR is a good indicator of this) that matters as far as your fitness is concerned.

Also, if you bought yourself a cheap bluetooth speed sensor then you might be able to use something like zwift or trainerroad to give you virtual power numbers when on the indoor trainer.

If your heart rate isn't controlled and escalates over a time period, then you're likely not working in the zone you're meant to be in. If you're unfit, then your power output will be less for a given heart rate. As you get fitter, you'll generate more power for that same heart rate. But if you do a 2 hour ride say, and you aim for your aerobic zone heart rate but it escalates over that 2 hour ride time then I would suggest you've set your heart rate target perhaps too high (but it could have been affected by other things too such as dehydration, heat etc).

Heart rate is affected by all manner of things (sleep, stress, diet, dehydration etc) but it can be used fairly effectively over a period of time as a way of gauging effort.

So really, aside from a period of riding regularly, I'd probably just head straight for your heart rate training plan.