I’d switch it to a 2+ hr, 3 if it allows it, outdoor workout, usually they give you the endurance front and back that would encourage steady state, and extend as needed.
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If it were me, I’d have been tempted to include sweetspot intervals within my long ride.
If you’re on the bike already … why not just incorporate some intensity.
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I agree with @gally24! I think this is a great idea, especailly if you enjoy the longer rides, and are comfortably managing that volume (which it looks like you are).
It wouldn’t be accurate to consider them equivalent per se, but they do both contribute to the development of your aerobic energy system. development. These specific workouts make up a small part of a larger picture. The sweet spot workout you are prescribed fits perfectly within the jigsaw of the training plan desinged for you. Whereas adding on zone 2 work here and there will contribute to some of the intended adaptations, but it doesn’t fit perfectly into the bigger pciture we are creating with your training plan. 
Prioritize the Workouts scheduled in your training plan for the best results. Then feel free to add on some Zone 2 if you enjoy it and feel like you benefit from it 
.
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Why not update your plan to a Master’s plan with 2 interval sessions a week. That way you would not even have to replace one. You can now set your weekly schedule to include 2+ hour outdoor rides as well.
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