You learn to ride rollers and away you go. Every year us roadies go through the ritual of riding either rollers or a turbo. I prefer rollers just because they are more traditional and I feel you get a better overall workout. You certainly keep your bike handling skills up to scratch.
They are hard to ride and normally it takes a few weeks to get into it. By then the good weather is back and its time for those rides back on the roads again.
I usually do between an hour and an hour and a half. If Im feeling really daft Ive been known to do two hours.
Riding in the big ring flat out for an hour takes it out of you but it just has to be done. My little shed is perfect as it keeps me cool but this year its been hard up to now with temps down to minus 10.
This picture was taken at Minus 7
So in sheds and garages all over the UK there are riders still training in even these conditions. One tip though, get a good selection of tunes on your I pod.