Catrin, what is your average speed and your average cadence? Those two numbers are important for your training schedule. Just for an ordinary century without too much climbing, a cyclist needs a minimum of a 12 mph average. When you bike distance with longer continuous climbs, speed going up the climbs becomes more important so that the average speed doesn't fall too critically low, so that you don't become a DNF due to not being fast enough. In other words if your average speed is 13 on the flats and rollers, but only 6 on the climbing, then your overall average speed will fall below 12 and you will have time problems. The cadence is important too because too low of a cadence requires more effort and tires the body faster; i.e. a higher spin is more efficient for distance.

Also, be flexible with your goals. I know cyclists who are naturally talented at climbing, who can do miles of continuous climbing with steep grades and they love it. Other cyclists dread climbing and the climbing takes so much energy they lose blood sugar and glycogen and suffer going for distance. You have to get out and find routes with at least 3-5 miles of continuous climbing, keep the speed up, and keep the cadence up. You might discover you hate climbing with a passion and there is nothing wrong with that choice. Some people are naturally gifted climbers, others have to work real hard at it, and some decide the pain is not worth the gain.

Another aspect of your goal is to consider how you want to do the long distances. For example, when I do distance my goal is always to be normal at the end of the ride, go home, have a nice dinner, soak in the tub, do chores and the next day I get up and there is no pain and stiffness. Other people do long distance and can't walk for two weeks.

And, some people do distance and decide that metrics is the distance they prefer. Just about every cyclist on a metric is capable of doing a century, but most of them prefer doing a metric. Century cyclists understand this point. You might finish a metric and realize that is all the distance you want to do, and that is ok, because if your life depended upon it, you could do a century in a pinch, but your preference is to do the metric.

I think you said your long distance now is around 30 miles. You need to change it and 30 miles is your short ride. When 30 miles is your short ride, and you rarely do a ride under 25 miles anymore, and your preferred distance is 40-50 miles, then you are ready for the metric. When your preferred distance moves up to 70 miles, you are ready for the century. You will know it because after every long ride you feel real good, and your perception of the terrain shifts, so that once what used to appear to be huge hills are just now slight bumps in the road.

And, train with a GPS bike computer so that you learn to recognize the grades, how steep you can go, when you need to shift down, and even what grades are just so steep it is smarter to get off the bike and walk.

Finally, there does get to be a time problem with all of the training. The cycling is fantastic, but most of us have only so much time we can spend on it.