dark fiber and sfp distance limitations

[mike-nanog at tiedyenetworks.com (Mike)]

I am looking at the possibility of leasing a ~70 mile run of fiber. I 
don't have access to any mid point section for regeneration purposes, 
and so I am wondering what the chances that a 120km rated SFP would be 
able to light the path and provide stable connectivity. There are a lot 
of unknowns including # of splices, condition of the cable, or the 
actual dispersion index or other properties (until we actually get 
closer to leasing it). Its spare telco fibers in the same cable binder 
they are using interoffice transport, but there are regen huts along the 
way so it works for them but may not for us, and 'finding out' is 
potentially expensive. How would someone experienced go about 
determining the feasibillity of this concept and what options might 
there be? Replies online or off would be appreciated.

Thanks.