40-60% is often listed as the 'comfort zone'. You want it low enough so that you don't get condensation on the walls or floor, and that may require it to be lower, or more insulation added. Any liquid water from condensation will promote mold growth. When you say it ran too long, there really isn't such a thing - they are designed to be able to run continuously (won't help your electric bill, but won't hurt the thing if it needs to - it will increase your comfort level). You will feel more comfortable at a higher temperature if the air is drier.
Make sure that the landscaping around the house is sloped away from the foundation and that the gutters disperse the water a ways away to minimize the vapor pressure of moisture on the outside of the walls getting in. then, if you have access, use some spray foam to seal all the rim joists and any holes to minimize air infiltration.