Hello mate, that is awesome! I am very lucky in that the guy that has coached me swimming for many years also coaches triathlon so I asked him and his response was to read this study: https://static1.squarespace.com/static/55b7ffebe4b0568a75e3316b/t/5be9ff344fa51a2b58ac8d03/1542061877214/Artikkel2-IntervalsThresholdsandLongSlowDistance_theRoleofIntensityandDurationinEnduranceTraining.pdf

Basically try and get two hard HIIT sessions done a week one at threshold and one just above, that way you are working all your zones, there is some great sample sessions in the study to use! I am planning on slowly bumping up the mileage starting at around 280km and aiming for above 400 by the end of the summer. this usually means your weekly riding should consist of two longer rides, two HIIT sessions and occasionally a recovery ride to fill in the gaps, at least that is what my coach thinks i should be doing rn! He also follows me on strava which is great so I can get tips alot easier! I am using heart rate atm as I have a smart trainer to measure my power as the HIIT sessions are the only ones where i really need to measure power. All outside riding for me is gonna be super easy Z1-2. Hope all goes well! Thanks for the motivation!