One thing I would like to add. For us old guys, which is anyone on this forum, 62 here, times on the same course can change quite a bit even with similar conditions simply because some days you feel better than others. Add in the factor of how well you fuel and hydrate your body on that particular ride.
Also, solo is tough on a century. I have ridden solo and I have ridden in groups. You will be much faster in a group because of drafting, along with the challenge of keeping up with the group.
Personally, I am capable of a 5 hour century and I am capable of a 7 hour century. Depends on how much discomfort I am willing to endure that day, along with the factors listed above. I always keep total time from the start until I finish, not riding time only. Also, my average speed includes the time at a stop. More realistic that way.