this winter for the first time, I had a wind trainer in the basement. I find that i cannot stay down there without a book. WITH a book i can ride down there for 30 minutes to an hour. After miles and miles on the road, i can sustain a fairly good consistent pace on the trainer, but without a book it is extremely boring.
When I went back outdoors, there was a noticeable difference in how quickly I got up to speed, so even though I was reading (and probably not giving it my all) it was enough to keep me in some kind of form.
My basement is cold especially in the winter, so i usually go down there wearing a sweatshirt or sweater over a teeshirt. Within 10 minutes I generally take the overshirt off. in 30 minutes I am usually a bit sweaty; and it no longer feels cold down stairs. I haven't used a heart monitor; but by this I can tell that I am getting a good cardiovascular workout.