OP not been back I see so perhaps wasting my breath, however....
I have to disagree with recommending 'Very High Intensity' rides concentrating on going faster for the OP who claims to be extremely unfit.
JayKosta I must agree with, don't worry about zones just ride your bike. If you wish to user a heart rate monitor to help you focus, then fine. My opinion and how I do it when starting from zero is to ride Zone 2 to build up a base of around 2k miles before I start looking at intervals and so on.
Zone 2 equates to what I have heard referred to as the 'Gossip Threshold' in other words at zone 2 you should just be able to hold a conversation if riding with another rider, if alone well I am sure you can work this out.
Les Woodward used to describe initial base training and how it effected your heart. First you need to increase the size of your heart and how much oxygen it can pump, so lots of zone 2 will do this and will increase your endurance.
However if all you ever do is low level riding your heart will end up like a large paper bag, no harm in this but it will not have strength.
An athlete who only does intense workouts will end up with a small strong heart a bit like a orange. The idea is to increase the size of the heart first (which is only a muscle and will increase in size like any other muscle) and then strengthen it with intense workouts.
So for the initial phase I would suggest you ride for the first month every other day at zone 2 or gossip threshold. (Only every other day to avoid injury, tendonitus etc)
Start at 20 miles each session, pick flat rides if you can. When you can do 20 miles OK pick every other ride up to 30 miles.
After a month start to ride every day if you can, perhaps with one or two rest days. Start to build up to 40 and 50 mile rides on at least one ride a week. All still at zone 2 or gossip threshold.
Pick flat rides for all of this where possible, or if you cant avoid hills, back off to keep the heart rate in zone 2. It takes some will power to do this. You have to be prepared to forget the ego and let any stronger rider go past and not try and hang on.
When you have logged around 2k miles then it is time to start upping the intensity. I do all the above alone as it allows me not to be carried away by a group ride. But after 2k miles you should be ready to start upping it.
If you can join a club and start riding the Sunday club runs they tend to be mixed terrain and you will find this an easy way of initially upping intensity and miles without it getting boring.
Its up to you how far you want to take it after this point, the main thing is to enjoy it