We present results from SEPPCoN, our Survey of Ensemble Physical Properties of Cometary Nuclei. This survey involves studying 100 Jupiter-family comets (JFCs) -- about 30% of the known population -- using the Spitzer Space Telescope for mid-infrared measurements of thermal emission and several ground-based telescopes for visible-wavelength measurements of reflected sunlight. The Spitzer observations (imaging with IRS PU and MIPS) are complete, and the ground-based observations (imaging in at least R band) are about half complete. Almost all our targets are imaged while farther than 4 AU from the Sun, to minimize (and often eliminate) confusion caused by dust from cometary activity. The Spitzer data constrain the effective radii of the JFC nuclei and thus give us an independent measurement of the cumulative size distribution (CSD). We find preliminarily that the CSD power-law slope is similar to what has been found by others using visible wavelength studies, suggesting that there is no strong trend of albedo with size. The Spitzer data also tell us about the JFC thermal inertia, and we find that many -- though not all -- cometary nuclei seem to have low values of this, consistent with a porous, fluffy, poorly-conducting, dusty surface layer. To our surprise, the Spitzer images show that about one-third of our sample appeared with extended dust emission despite being close to aphelion, and in many cases the dust originated from cometary activity happening right then. Interestingly, we find that the activity observed at these distances seems to turn off rather abruptly once a JFC passes aphelion. We have used dynamical analysis to constrain the dust grain sizes and thereby distinguish dust tails from dust trails. The dust temperatures are in most cases consistent with isothermal, low-albedo grains in LTE. We thank the Spitzer Science Center for supporting this research.