Inner ring 42 tooth count ...

Are you young and strong ... or at least well on your way to get strong?

Go first with a 16 tooth cog 3/32nds of an inch wide. Keep both brakes for a while.

That is what I did and I cannot complain .... love the thing. The horns will help you position your body for enhanced climbing, so the lack of gear will not be as critical.

Local/friendly bike shop can do it for you, or do it yourself ... you gonna love it.

While you are at it, get yourself some MTB shoes and pedals. Safety and efficiency in one package.