I have a Lennox air conditioning unit that is about 20 years old. It has worked okay for the most part, but just the other day we noticed that it was running a long time before turning off. It does keep the house at the setting we put it at. (78). The concern was the lines from outside had a lot of condensation on them and where it enters the coil area there was a chunk of ice. Outside the lines had frost on them. Could it just be low coolant? We did have a leak fixed about 4 years ago.

Sounds like you have a small refrigerant leak causing the frost up situation. Best to call a pro for a ON SITE inspection and repair. You can also check your air filter to be sure it is claen.