i would say dont overthink it. you will gain a lot from your schedule been sustainable and if this mean you using commuting as training... it s a very smart move and it s very effective.

What i have done with pass athlete using this strategy is doing a steady ride to work.... and putting a short main set or big gear set on the ride home. We get good quality and make you work hard on the way back. It works, it s smart time management and as long as you get the time in....dont worrie too much about the differences in stimulus of one vs other....

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Jonathan Caron / Professional Coach / ironman champions / age group world champions
Jonnyo Coaching
Instargram

I think the real question should be: can you fit in 1.5 hour rides during the week? If you can ride easy to work in the morning via the direct route and then extend your afternoon ride home to an hour or more, then I would say that is a better option. If you absolutely can't extend the afternoon ride because of other commitments that are more important, then go with jonnyo's suggestion of putting a quality main set in the afternoon and don't even think about what a 1.5 hour ride would be like (since it's not an option).

The most important thing in my opinion is to do something that is sustainable in your life right now, so do that and don't worry about whether the unsustainable options would be "more effective".