Perhaps I am confused, but you are outlining a process totally different from the actual “Alternates” process TR created and documented.
Have you tried that or found it not to work? You can alter Duration (time) through that process.
Your ideal (setting time limits) is something TR wants to do as mentioned in this Feature Request and the podcast via Nate. But it’s not part of the process now. So using the Duration side of the Alternates tool/process is the easiest option available at this time.
It looks like alternates are now available in the web browser version! I was pretty excited to find this since I’m just starting a LV plan but will likely move the 1 hr to 90 min workouts as my schedule allows.
I should add…the “replace” button is grayed out because I didn’t select one of the alternates. It works as expected once a selection is made.
Well fancy that! I’m sure that wasn’t there when I last looked a week ago lol. Maybe I wasn’t looking closely enough 
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@mailman yes, we just improved Workout Alternates on the website a couple of weeks ago! 
Sorry about the delay here, @Yo_Adrian! I’ll try to explain how you can use Workout Alternates from both the App and Website.
Workout Alternates from the Website ( TrainerRoad.com ):
You can now find Alternates directly from your Calendar.
See the screen recording and screenshots below.
Workout Alternates from the Apps:
As @mcneese.chad mentioned, to see step-by-step instructions on how to use Workout Alternates on the desktop and mobile Apps, check out this article here: How to use Workout Alternates in the Apps.
Good luck with your training! Let me know if you have questions about these steps.
I’ve been on low volume plans for the past three years and it’s worked really well, I generally approach 300w FTP in the summer, and at about 70kg, that’s comfortably 4wkg.
However since my 3rd child arrived in December, my evenings have been constrained (this child screams when not being held). So I have maybe half an hour of riding I can do perhaps four times a week during my 1hr lunch break.
It’d be great to slot in my available time into the calendar, and for the programme to give me a plan built to make best use of it.
Would be awesome.
Nate mentioned this in a podcast a few weeks back. Here is the thread with discussion.
Yes please do this, as I can do a little more than the low volume plan but not always do a 1.5-hour ride on the weekend, would much prefer to set something like 5 hours and let Plan Builder do its thing as opposed to the 3.5 hours but then have 1-hour workouts instead.
I know I can use TrainNow or supplement workouts with this, but it is not as fluid for me. I just wish to have a plan and follow it as best I can.
Nate mentioned on the podcast that this is on our roadmap!
Stoked to get this feature out, we’re just focusing on some higher priority releases first. 
Thankyou 
Sorry for the bump, but this would be an awesome feature.
The answer is prob no but wondering if there is any update on this feature in regards to potential date of release?
Hey there! We don’t have an ETA at this time. We’ll be sure to post any updates that come in here, though!
I bet most of you in this thread have seen the updates already, but in case you haven’t…
Custom Workout Duration for Plan Builder has gone live! 
