I suppose a lot of the answers to the questions you’ve got only sort of make sense when put in the context of “what were training plans like before” - but since that’ll take a bunch of words, I’ll stick what I can dredge up from my mind and stick it in this expanding spoiler tag, before attempting to help underneath!
Previously on Zwift workouts
Previously; when you signed up to a training plan, each workout would only unlock at a certain point in the week (for instance, workout one would be available on a monday, the second maybe on a wednesday); if I recall correctly, you’d also only have 3 or so days to complete each workout in a week before it was “locked out” and unavailable to choose - basically the available workouts for plan would always keep progressing, whether you completed them on time or not.
In 2021 or so, some changes were made to improve flexibility of training plans - this made all of a week’s workouts available for selection on the monday, with the only requirement being they needed to be completed by the sunday night of that week - this also had the slight downside that what order the workouts appears in, didn’t always match the original order they were designed to be done in. (Plans with a race finish are usually a good example, with “race day warmup” being earlier on the list than a “final taper” workout). Though I’ll mention this in point 3 below.
Then in the last couple of month we’ve been given the ability to complete a plan with an extra 2 weeks of flexibility in addition to all the other changes; this does mean that a lot of the “do by” stuff is totally superfluous, hopefully there will some efforts to address some of the unneeded hassle of “do by whenever” getting removed, and having the workouts in the “right” order once more.
I’ll caveat these answers with “I’m not a coach, I’m just here to provide a vague answer to the questions you’ve asked”.
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Wednesday the 3rd of whenever, as mentioned on Rowdy’s reply that I’ve linked to in the first line; these “do by” dates are totally redundant now. You get X+2 weeks (where X is the number of weeks you set the training plan to last for) to do all the workouts in and complete the plan - there is no requirement to get specific workouts completed within a timeframe (other than the overall X+2 weeks to complete everything).
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As long as you’re completing a week’s worth of workouts each week (and those workouts are being marked as completed in the training plan - I’m not sure if there is a requirement to complete/star a percentage of segments within a workout in order to mark it as complete), I wouldn’t worry about it.
If you’re unable to get enough workouts attempted/completed each week (due to not being able to fit enough sessions in to complete enough workouts for instance), you’ve still got an extra 2 weeks to complete the plan - if that’s not going to be enough (for instance, you’re on a 6 week plan, with 4 sessions, but you’re only getting 2 workouts completed a week; you’ll end up needing to cram more workouts into the extra weeks then you’ll have previously been doing on the bike), it might be worth looking at another plan with fewer weekly session. If you’re wondering what “a week’s worth of workouts” is…
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The workouts are “probably not” in the correct order - thankfully Zwift workouts and training plans | What's on Zwift? has a list of each training plan’s weekly workouts - and their original order. It’s a little frustrating needing to print out a list off a 3rd party website in order to get some structure back to each week’s plans, but unfortunately it’s where we are.
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There is a bit of blurb on the workout plan chooser page that may give a flavour of the plan’s goals. Personally, I tend to choose plans based on the following:
a) have I done it before (as I want more virtual socks)
b) how many sessions are there each week - does that fit with my current schedule?
c) how long is each session - does this also fit with my schedule (are the sessions similar lengths, does that length fit my schedule; are there requirements for longer rides that may/may not fit?)
d) what do the sessions actually look like (see whatsonzwift) - do they favour long stretches of steady state (FTP builder/fondo plans) or are they a little more “busy” (build me up) - and whether either of those are something I’d prefer.
I’d also mention that the “difficulty” of any given workout is entirely in your control - a woorkout’s segments are based off a FTP value you manually set; with ±25% worth of adjustment available during the workout to make it harder/easier as necessary (though I’d mention that some plans feature a lot of “easier” riding, there would be a temptation to jack the bias up at this point for more of a burn - that’s not necessarily a good thing).