Training volumes for 70.3 followed by 140.6

My ATP includes a few 70.3 at the end of 2019 and early 2020 followed by a full IM in July 2020. Since this is a long term plan, when should I begin ramping up the training volume for the full distance as opposed to the mid-volume?

Some background on me: I am a recent convert to TR and love it so far. I have been using a competitors program but I recently came across the TR podcasts which honestly is what pulled me into TR…great program!!! I have completed several full IMs several years ago. They were mostly about survival; however, my goal for this full IM is to be much more competitive.

In the last year I have been much more consistent with some indoor training mixed in and feel much stronger than back then. My last 70.3 in December was 5:04. Currently I am at about 3.5 watts/kg and want to build that as much as possible….hence the TR subscription. My motivation is as high as ever and love the process of training so no worries there about the commitment. I just want to know when I should start incorporating the full IM plans over the half IM. Should I wait until later 2019 or is there benefit to building a much stronger more durable base now?