There are many times when my legs don't feel good till 45 minutes or more into the ride. However in my data, I was doing better during those times than my perception would have me believe.
I've heard some say if your legs don't feel good then don't ride. And that might be true for instances where you know you have not been getting proper rest. But in general, I know I've gotten proper rest and just ride anyways. I almost always am surprised what my numbers show during the lengthy period that they felt like crap, and even more surprised at my numbers when my legs are finally feeling great.
It almost is to where I hope to have crappy feeling legs for almost the first hour, because when I do, they feel great at hour 3 or better. But remember, I did say "almost". <grin>
Does this relate to being on a trainer? I don't know. I still can't bring myself to get one, though I do sometimes use my wife's stationary bike to keep my cadence up when I can't get out in the real world. We just don't have enough days stuck inside here.