Awesome that you are doing an IM! - your first?
I agree with other opinions, here. Training for an Ironman (or any cycling race) using “miles ridden” as the main bike training metric is never going to end well. Kind of like the swimmers you see at the pool, swimming the same speed non-stop for an hour every day - yes, they have a great base, but won’t be getting faster.
Like Mark suggested, TSS is a great tool, re. how hard was the ride?, how much rest do I need? How does the ride fit in with the week’s training? BTW, are you currently using Training Peaks? I am a triathlete myself and also coach 4 Ironman distance triathletes. A power meter is compulsory for any athlete that I coach (zwift, too because I believe that if you really want to TRAIN and not just RIDE, training needs to be done with power. To me the most important 2 ways to spend your triathlon $, are a power meter and a professional bike fit (most triathletes I see in races have awful fits and then wonder why they have issues running). Also, z power is OK for a brand new person to riding/zwift but that’s about it.
I think that if you invest in a power meter, do a proper FTP test and follow some good workouts on zwift, you will probably see a noticeable improvement in your cycling performance - it’s a lot of hard work, but fantastic to see real results. One of my female athletes went from an initial FTP test of 148w 2.2 w/kg, at the beginning of September, to 228w 3.4 w/kg, in mid January. Up until September she had been distance and HR training only. She has worked very hard and tells me that she hates me now and then but is going to be an animal, this season. Now, every ride has a purpose, with no grey or “junk” miles. I also believe her running has benefitted from this improved bike training.
All the best, for your training!