It adds up, assuming the building is more or less rectangular, there is about an R value of 2 in the walls, and it has some insulation in the attic (R-20 for materials plus insulation). See this simplified heat loss worksheet (simplified meaning windows and doors are not considered because the walls are uninsulated to begin with, the heating season is simplified to 30 ASHRAE 1% design days for Chicago for a ball park load estimate, and natural gas is priced at $13/MMBtu which is about three times wholesale at a gas distribution hub in the US today): http://aatcons.com/~diesel/heat_loss_kurt.xlsx This also assumes the building is exposed to a temperature differential of 70 degrees on all four sides and the top for the heating season, if one or two of the sides of the building abuts another building, or if you actually use heating degree days for this facility you may get a lower loss and associated cost. As to why the system heats so quickly, the thermodynamics in question are the thermal conductance of the iron in the radiators (which are good - the iron is conductive and the walls are fairly thin), the specific heat of the water vapor in the pipes (water vapor can carry relatively a lot of thermal energy per unit mass), and the thermosiphon effect that helps draw the water vapor through the pipes to the radiators. The boiler fires and may make a little steam internally, but once the the thermosiphon starts most of the material actually flowing through the pipes is hot water vapor. You have a very well balanced system, Kurt, so the optimal volume of water vapor for each radiator is flowing simultaneously, which is why the system doesn't make noise but does heat up quickly. Finally, a lot of people don't really understand these systems and assume incorrectly the cast iron radiators are inefficient (they are not - see Kurt's narrative), or that simply increasing the boiler efficiency will have a huge impact on fuel consumption (it will not - again see Kurt's narrative on overall system efficiency). A $10K condensing boiler install is paid back in 20 years in Kurt's scenario if you plug a bump from 80% to 95% into the worksheet I linked above.
