Start with the obvious. Are you sure your odometer is accurate? Get on a highway and watch the mile markers. Go as far as possible and verify that your odometer agrees with the mile markers. A GPS obviously will work too. What size tires do you run? Have you made any gearing or transmission changes?
If that checks out, check the basics of the engine. Get a compression tester and PROPERLY check the compression. It should be even across all 6 cylinders and somewhere north of 150 psi. When you have the spark plugs out, take a look at them. Are they light tan, of black and sooty?
A vacuum gauge will also give you an idea how it's running. Pull one of the vacuum hoses off the intake and hook up the gauge. If it doesn't read 18 - 20 inches of Hg at idle, the engine might be getting tired.
My 10:1 CR '78 280 gets 23/25, so yes, mid-20's are very possible.