What you have is not half step gearing. Half step gearing would be something like 49/46/28 or 50/47/28. With half step gearing, most of the shifts are happening at the rear, unless there are hills, when you shift on the granny up hill and the big down hill. I am not a big fan of half step, but there are people who swear by it.

As far as the rear cog goes, if I was building a long distance touring bike (and you got an excellent one) , I would look for a 7 sp rear freewheel, with at least one bail out ring (like a 34T) and add an RD that can shift that (most long cage 90s Shimano RDs can do that.) This set up with your 28 granny up front works pretty nice on nasty long hills. To make this happen, you got to use friction and the suntour barcons are probably the best way to go. And, since you live in San Fransisco, you would love that 34 with those hills

Another thing is that you might want to think about is changing the bar into a moustache or flat bar. Barcons will work on a moustache bar. If you want index shifting, you might want to look into putting MTB parts. In my long distance bike I am riding a 46/36/26 front with a 12-28 7spd, with a flat bar (bar ends and aerobar) and MTB index shifting components. I find that to be a very happy combination for long rides (for me) and still having decent gears to speed a bit if I feel like it, plus the set up to ride in the aero position down hills (or when drafting behind someone) and just glide...