Training for long distance rides (> 200km)
While gym and spinning are good complementary activities to cycling, I would recommend that you also look at adding an element of riding on a real bike (like the dark knight said in his comment).

For long distance road riding, there are a number of things you need to practice.  In my honest opinion, the two things that need the most practise are: 


Sitting down in your saddle for a long period of time - 200 to 300 km will see you sitting down anywhere between 8 and 18 hours (depending on ability and terrain). That is a VERY long time in the saddle even for an experienced rider with the best type of shorts / saddle.  
Nutrition - One does not simply ride for more than 3 hours without a tried and tested nutrition plan!  Your body usually stores around 90 minutes worth of glycogen (at Tempo effort). This means that around that time, you should have already eaten and digested food, ready to be burnt up by your muscles.  You should top up these reserves as often as possible, so that you do not go into a minus and become hypoglycemic (bonking).  I usually eat a bar every hour on the hour, sip carbohydrate drinks every 15 minutes (700ml an hour) and I top up in between with gels.  A typical 4 hour bike ride for me burns 3500 calories.  Pro-riders easily go through 8000 cals a DAY!


In addition to the above, it's very important that you have a bike that fits you properly.  I'd recommend having it professionally fitted because you feel all sorts of pains after sitting in the same position all day!

If the weather is really as bad as what you say, then you could consider getting yourself a turbo-trainer or rollers.  You'll get used to sitting on the bike you'll be riding, and provided you have the time, you could also practise your nutrition and eating and drinking on the bike (if required).