all of the above
and, if you're getting dropped in the rides with your ride buddies (with whom you plan to do the 100), then either work hard to stay on a wheel in your current rides, or plan to 'hook up' with other informal groups on the century... not disallowed if you ask and also become part of the group. In fact, if you're dropped along the century, it's not a bad idea to keep hooking into small groups, progressively, after getting dropped... Usually the 'groups' become slower paced, the further into a century - fast riders go out harder, finish faster - and so on, as a century progresses.
...doing a century, by your lonesome, can be a real mental challenge for anyone who has not ridden near that distance...
if you're not comfortable riding close in a steady group (no idea how you're group 'rides'..., some can be quite inattentive chat sessions). then that is a good objective to develop as you progress towards the century.
There have been many great threads on BF covering group riding... also good info on the interweb.
Ride On
Yuri