Old unit central A/C questions..

20 or 25 year old central air unit here.
I took a fairly accurate insta-read laser thermometer and discovered that the temp coming out of the ducts is about 60 or 62 F in most rooms (I'm in Canada but I still use F) House is set to to be cooled to 76 when we are home and 73 at bedtime. When we are away house is set for 83 F. Humidity right now is about 55% inside.
So If the air about 4 feet up is 76 F and the air coming out of the vent during cooling is 60 or 62, is this still reasonable?
Looks like the refrigerant run from the a-coil/furnace to the compressor unit outside is 18 or 20' including bends. Insulated. This a nice short run? I notice in other homes about a 40 to 50' run. Does the length of run make any difference?
What effect does dust or dirt have on the blower during a/c season?
Any use running the blower all day on a hot day to keep the air circulating or is that a waste of energy. My wife insists no ceiling fans whatsoever. I've argued three times and she still insists no ceiling fans.
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