Congratulations--it sounds like your are a good rider already. While I am a huge fan of outdoor riding in all sorts of temps (but not rain), I found that taking a spinning class once a week really upped my fitness. In the winter, I take spin twice a week if I can manage it, in addition to outdoor rides.

Just keep riding and you will get stronger. Some clubs have cue sheets for rides of varying lengths. You'll need a cue clip and a bike computer to follow the directions, but you won't get lost (at least not as lost as without a cue sheet).

Have fun!