Here's your Soho S, if my search turned up the right bike -
http://www.trekbikes.com/us/en/bikes/urban/soho/sohos/
According to the page, it has 28c tires on it. If it has held up to whatever you put it through so far, I'm sure a road bike would be fine. The road bike frame isn't going to be a problem. One question is if it comes with less durable wheels than your soho - that's about the only place that causes "fragile" problems. The other question is if it has clearance for 28c tires - frankly though, pretty much all road bikes take 25c tires, and if you're happy with 28c tires if you had to go to 25c it probably wouldn't be that huge of a deal.
I own a bike with an IGH. I wouldn't get an IGH for your trip. And IGH is heavier, has larger gaps between the gears (not that that would be a big concern for you, though), is more expensive, and isn't as efficient as a derailler. I use it as my winter bike - I wanted it to be my summer commuter as well, but it just wasn't as sprightly as my regular road bike (a $800 bike). There's no way I would do a 50 mile trip with it unless time didn't matter (like on a weekend trip or something). It's not "terribly" different, but over 50 miles...it was enough that I bought a regular road bike for summer commuting.
One thing I don't like about the Felt is that there's a lot of drop between the seat and the handlebars (in the pic at least). Of course seat position depends on how tall you are, what size bike you get, etc, but I've found if there's a lot of drop it can make finding a comfortable seat a lot more...difficult. One where you don't have to worry about...numbness and rubbing. Of course I don't know it this would be a problem for you, but they do make nice road bikes with a less large drop between the saddle and the handlebars.