Winter training

At least here in Finland the autumn is in full swing, the winter is closer and suddenly temperatures have dived significally. Not to mention that many paths and roads are covered in wet leaves. So, time to start thinking about winter training, because I want that first 200 km brevet to be as painless as possible. Now due to weather and road conditions my road bike stays indoors and will be attached to a trainer (a dumb one). How often and how long should I do indoor training? On a good week I may still ride my other bikes 12 km round-trip to work and back five times a week, but that hardly constitutes a training programme.