Gearing Down for Courage Classic

With the Courage Classic just a month away, I've started wrapping my mind around the fact that I'm going to ride 170 miles, over the mountains, on one gear.  For my training rides around Tacoma, I've been riding 36x18, which is about 54 gear inches.

Depending on how you ride, this gear might seem too small,

 or too big. 

I think this gear will give me the best range for climbing the hills, cruising the flats, and pedaling into the headwinds on the descents.

As a back-up plan, I will likely thread a 20T freewheel on the other side of my fixed/free flip-flop hub.  That way I can just flip the wheel around and have an easier gear for the climbs if I simply cannot turn the cranks.  The rear brake may have to be left open for this to work, but I can then flip the wheel back to the 18T side when I reach the summits.  I hope to do the whole ride without flipping the wheel, but I'm just not sure which one gear I need.

Any thoughts? Have you done a mountainous road ride with one gear?