For just getting into cycling, yes it's respectable. Ride consistently and train and at your age you could be at the 5 hour mark soon. As others have noted, elevation makes a big difference.
I'm 61 and will never do a 5 hour century, but that's OK. A few of mine for example:
Miles Elevation Time
108 11,437 8:40
104 10,538 8:24
100 3,140 5:53
100 5,295 6:17
Wind and other weather factors also makes a difference. Rain or heat can slow you down a lot.