My goal is to up my weekly mileage to 150. I’ve seen it said on SYSTM that a 1 hr HIIT ride is equal to several hours in a group ride. Is that correct? And, is there a rule of thumb to estimate the number of road miles a SYSTM HIIT is?
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Tl;dr: That Dependz® on soooo many factors.
Welcome to the forums @TomBoro. There is no rule of thumb that I am aware of and I’ve been indoor training with SUF/SYSTM (as well as Zwift and other apps) for the better part of the last 10 years. That said, I do recall one video maybe it was GOAT or Power Station that says it’s like the equivalent of a multi hour ride because of the strain it puts on the muscles.
Here are just a handful of things to consider:
- Is your group ride on hilly, mountainous, or flat as a pancake terrain?
- Are you typically riding in a large group where there are plenty of opportunities to sit in the draft, or are your group rides a handful of folks where the work is more evenly distributed, or something in between?
- Are your group rides spicy like the World Road Championships every single time or are they more measured and controlled with only a couple of clearly defined sprint points?
- Do you find yourself completely gassed at the end of your group rides or do you feel you could probably go for longer if you had the time?
- Etc etc etc.
So, yes, a 1 hr HIIT session (depending on which one and what that session is targeting [eg. sprinting? endurance? aerobic capacity/recovery?] are “generally” going to wipe you out a lot faster than the same amount of time on a “typical” group ride (whatever that means) and it’s generally NOT recommended that you do these types of rides daily (as you need to build recovery into your training).
My unsolicited 2 cents, if you are short of time, but want to improve, then pick a training plan (assuming you’re using SYSTM) with a low to moderate volume and you’ll get a good mix of HIIT, recovery sessions, strength/yoga, and mental training that will see you improve in most areas by the time you’re finish, provided you’ve been consistent (about 70-80% compliance).
Edit: I should ask if, when you say mileage, do you mean miles or kilometres? And, are you looking to spread that mileage out evenly or do the bulk of it on the weekends? And, what is your goal with that mileage? Is it to get it done faster, or is it just to get it done?
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Sir @Glen.Coutts makes many good points.
My opinion is that mileage/distance is NOT a good way to relate indoor trainer miles to outdoor. The key factors are intensity and time.
Having said that, the loose rule of thumb I’ve always used is 1 hour on the trainer = 1 1/2 hours outdoors.
There are so many variables that I don’t feel a reliable, direct correlation can he made.
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I guess there is a rule of thumb. 
Fwiw, this won’t be the only time I’ve been proven wrong. 
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Guys, to continue. Your points are well taken. A conversion factor would be a rule of thumb or “wild ass guess” as we say in the south. However, I don’t want to short myself when I am training in doors and outdoors.
Thank you for your helpful responses.
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There are factors such as fueling, pacing, weather (heat, wind, etc.), etc. that are present outdoors that are not present indoors. Intensity Factor indoors does not easily translate to outdoors.
Not to worry Sir. After all, as I recall, you’re not even a botanist, so how could you have known ? 
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