Cranks forever before starting

I have a Courage 20 in a 42 in ZTR Cadet. Had trouble starting so I changed the Spark Plug-Air Filter- Oil.
Also changed gas filter It cranks forever before it starts....I'm afraid I'll burn the starter out. I did open the gas tank cap
...still takes to long. Any ideas?? It also backfires if I don't let it idle down for several minutes. Bill

Cranks forever before starting