Way to go Stap!

When the alternative is to do nothing all winter, using the trainer or spinner is a great choice, especially if you can make the sessions short and intense. But you are training muscles more than the heart in doing this. I would hate to try to maintain an aerobic/endurance base using the trainer all winter but, fortunately, that is the component of overall fitness that declines the most slowly.

But the endurance does still fall off some. I finally got in an outdoor ride of 1:48 last week (for the first time in about a month) and was a little more tired than normal afterward... but I was strong during the ride. When the time comes, the first few longer outdoor rides of the year are still going to be tiring and I think you should expect that too. That doesn't mean that your trainer work was wasted though. Ride on. It's the right thing to do if you can manage it.

(And a little core work in the gym doesn't hurt anything either.)