It's hard to say about the legs. I read that when riders switch from upright bikes to recumbent (on the road) bikes, there is a considerably time where they're building up new muscle. In your case, that might relate to the type of indoor training you're doing, not the fact that you're doing it.
Two other issues come up. One is that if you actually ride outside, you get more used to dealing with wind, hills, hot, cold, etc. Part of that is mental, part of that is physical. The other issue is motivation. I have zero desire to do any kind of indoor riding, so it'd be hard for me to get in enough time doing it.
Do people riding trainers ever ride "into the wind" for 4 hours at 12 mph? Just wondering, as that's a part of it.
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"be careful this rando stuff is addictive and dan's the 'pusher'."