This one. 
You can’t do intervals/intensity every day. You’d die - eventually. What you can do is easy Z2 between your hard days.
As you get aerobically fitter (as a result of all your riding) your cruising power will increase. Eventually, what’s threshold or sweetspot now will be your Z2. It’s pretty cool when you get to that point.
The problem with cycling, or any fitness sport, is that there isn’t just one thing you need to do to improve, and lasting improvement is managed after time & not done in the short term. Yes if you’ve just started you will make gains, but then you plateau & that’s when you need solid training plans & a good diet to go to the next level. In the Army we called it the never ending quest for total fitness.You might hear mantras trotted out by fitness people such as “abs are made in the kitchen” or “ you can’t out train bad diet”, they use them because they’re true.
Leg strength is important as well, so is core strength. Both of those need off the bike workouts.
Core strength you can just go look at the workouts of Max Whitlock OBE on his youtube channel, they are time effective and work. He has done quite well at the elite level of his sport.
You can spin around at Zone 2 all day and you’ll have a great Z2, but for the rest you need to switch things up, and let the recovery happen as well.
Do your easy days very easy, otherwise you’ll just end up riding around in the dead zone and never improving, always feeling fatigued.
To start with, more distance and more regular riding so that becomes normal and gets easier, then you can think about changing things up and starting with specific training blocks.
Did my first Zwift race today - 34 minutes at 167w, and Zwift estimated my FTP after at 165w, which isn’t far off what the ramp test gave me…I’m assuming my FTP was calculated correctly the first time?
Now you have a target to aim for. Most of my FTP increases have come from taking on the various routes in the climb portal, I did them regularly as I was going for the climb Mount Everest challenge & the shorter ones are pretty good for increasing your FTP.
That’s a good start - for 2 weeks of riding as per your other topic.
Spend a lot of time on Alpe du Zwift as well, use your gears (and not ERG) and you’ll start getting stronger, you’ll learn to pace yourself correctly and how to vary your cadence with the gears to balance heart rate and your legs burning. It reflects real world climbs where you don’t have the ideal cadence all the time because the gradients change.
Higher cadence easier on legs, puts more on cardio, higher gear slightly easier on cardio but your legs burn more. All this is good for real life riding if you are riding up longer hills.
With the exception of when both don’t work and you are stuffed! 
I’m laughing because I remember doing FTP tests early on before I got fit and in that situation with 8 minutes still to go… (oops).
It’s all learning what works and what doesn’t, and it doesn’t get easier, you just get faster. 
You can also get stronger with ERG mode. Both are fine, there are pros and cons to both.
Honestly, climbs are my kryptonite in real life - even spinning to win it in the lowest gear skyrockets my heart rate, but I’ll try doing the climb portals as well, especially if that will help me in real life.
I’d love to ride on max trainer difficulty, but it gives me some knee pain so I’ve been turning it down, which feels a bit like cheating, but I guess that’s how it goes…
Managed to even get a green sprint jersey today on a popular route (Flat Route) and kept it for almost 40 minutes, still having 3rd place when the 1 hour ran out. I’m getting better, which is nice. Ran with the 1.5w/kg group today for 2 hours. All I have to do is lose some weight and I’ll be doing 2w/kg easy.
I like ERG mode as it helps me pace myself. For my metric century, I did 3 hours @ 100w in ERG, and the last hour without ERG. I feel the ERG helped keep me fresh and allowed me to zone out of the pain and not pay attention to the pain and suffering!
I don’t know what weight you are or how much you think you need to lose but as I’ve said before, make it a long term goal. Putting a massive strain on your heart won’t help you at all, as my brother found out. When he decided to lose weight he just took off on a bike to work effort every day, including a short sharp hill going home. He’s now the owner of three stents after getting chest pains & falling off his bike, on day four!
Jeez, was he active at all? How much did he way? What was the exact diagnosis?
I’m in decent shape, in the past 2 years I’ve done 2 half marathons, regular running and cycling, + weight lifting. I’m not super overweight, 85.5KG @ 166cm, a lot of that weight I gained from a medication I was taking that causes weight gain as a side effect.
The fool was close to 300lbs & hadn’t done any exercise for years, he was a good rugby player back in the day but did his ACL in & never did much again.
It’s just a cautionary tale I tell people who are keen to get slim & fit as quick as possible, but as in most things that are worth it it takes time.
Also cautionary tale to get a stress test in a clinical setting before doing really hard efforts since there are lots of ways to die, not all associated with being unfit.