Hi I’ve been on Zwift for years, using it for long distance for my previous ironman’s.
I’m now concentrating on 70.3 and below, i currently ride around 18mph and have got 6 months to get 20+mph.
What would you suggest to use, doing 3 rides a week, thank you
Can you afford to get a coach, or use TriDot if a coach is not affordable for you?
The speeds on Zwift are not comparable to those irl.
There’s no weather to factor in. No corners or junctions.
You’ll be better off working around a power output as opposed to speed.
Also, position on the bike, aerodynamics of the bike will matter a lot, and ability to sustain power in that position - things that don’t matter indoors.
If you’re staying on here, as opposed to irl riding I would suggest 1 long ride & two sprint type sessions. Have a look at the VO2 max rides as a way of increasing your basic fitness level. Just doing long rides, even over & above your Ironman distance, will only get you so far & doesn’t usually increase your pace too much while sprint, or short but intense efforts does.
If you are on a TT bike / tri bike then position on the bike can make a surprising difference. By keeping yourself as small as possible, dropping shoulders, tucking your head lower, within limits of what is comfortable and safe for you. David Millar (the former pro cyclist) had some tips on that on a YouTube video. I was also applying them myself (riding Cervelo P5 UCI legal TT spec) and he was right.
The other gains will come from leg strength (gym workouts), then the aerobic capacity.
Sweet spot workouts will help bring up that, but they are quite miserable to do. 
These are typically your over and under efforts, start over FTP, then hold just under FTP for extended time, the rest, repeat numerous times. You also need to mix it up with easy rides, otherwise you won’t get the improvements from your training - just riding around all the time fatigued.
I might also suggest trying an outdoor velodrome if you can use one so you can get your outdoor training (particularly getting used to aero positions) dialled in without interruptions from other people or worse - cars since you’ll be on the aero bar extensions a lot and don’t have brakes available quickly. The velodrome gives you some safety. In my area TT bike efforts are a bit too hazardous now, things are too busy.
Before I built that P5, we set up the same position and geometry on a fixed trainer bike frame and I did some power tests to see if I could hold the position comfortably and what power I could make. That’s another thing you can try particularly if you have something like a kickr bike.
My other way, which is difficult - spend a week riding in the mountains at higher altitudes. I’m always flying when I come back and ride at normal elevation. It lasts for a few weeks then eventually fades off.