My commute is half the distance of yours. But I've done it with gears and in SS so I have some basis for comparison between the two. The key question is whether you have lots of steep hills and/or relentless wind. That's what gears are for.

Absent a hilly or windy ride you will find SS not just doable but better. If you pick a gear that fits you and your ride - which may require a bit of trial and error, or perhaps different gearing on two sides of a flip-flop hub - you will not be in a wrong gear 80 percent of the time. On the contrary, relative to a derailered bike, you will be in better than the right gear much of the time, because you aren't running that gear around a pulley system, and dragging the weight of a bunch of other gears you rarely use and don't need. Not to mention maintaining that system and being serenaded by its many noises.

I live and work in two cities with a geared bike in one and a SS in another. I am currently scouting for a second SS so I don't need to commute with gears any longer. I love my geared bike...for rides and conditions that call for it, or as a change of pace. But it's not the right tool for a mostly flat daily ride through the city and up and down from my fifth-floor office. It's heavy, the machinery is not great in the Wisconsin snow, and over so many miles it requires too much tweaking and care, especially to maintain the whisper-quiet ride SS has spoiled me to. I'm trying to clear my head to and from work and it's a relief just to ride in silence and not be shifting and trimming gears and worrying about cadence all the time.

As for FG vs. SS, I'm just now starting to experiment with FG. Over your distance you'd have to be in great shape to go FG. If not you would get there fast.