What gear should I be in?

Not your gear that is the issue but the engine.

Keep riding more and spend lots of time in the saddle and you will get faster.

I think you answered your own question: " I am not a great distance cyclist, I am a long distance runner and in good shape."

Being good at one sport doesn't equal you will be instantly good at another.

Ride more and ride faster...... that is the only way to get faster. Until then, you and I can bring up the BOP.

Do you have cadence on your bike computer?

Pick your gear based on your cadence. I like to keep mine at 90-100. The faster I pedal, the higher my cadence.... I shift when my cadence is over 100.

It's not about the gear, it's about cadence and moving your legs to keep the speed at the proper cadence.

Well, part of it may have been that I had been mentally preparing myself for the entire 100 and didn't want to burn out, I tell myself that anyway as people go by me.

You have to hammer the road at least 3 times a week. The more you do it the better you will get, same as running but different

In most large group rides like that you'll usually find a group that you can hang comfortable with.  It sure makes the century ride more tolerable.

Some people are faster, some people are slower. Gearing is not the major factor.

The cadence suggestion is a good one...there are bike computers with cadence functions that aren't very expensive (I think Cateye has a very cheap hard-wired one...I have a Sigma wireless one that was only like $90).  In general terms, keeping your cadence around 90rpm is a good starting point (at least for triathletes...it mirrors a pretty optimal running cadence)...shift your gearing until you can maintain something around that cadence comfortably.  You may find that you were trying (unsuccessfully) to push too big a gear--i.e., not maintaining a desirable cadence--or that you were at a good cadence, but just don't yet have the bike fitness to ride at that cadence in a gearing that more fit cyclists are able to.