After three years, this is your pace and if your knees aren't bothering you, then just work on daily consistency. I also am not sure about running and knees. I'm "only" 54, but I've been seriously bike commuting for 24 years. If you feel "stuck" try varying your routes, or buy a bike (or bikes) of a different riding style and see how that works. I have three bikes and I try to mix it up to keep from repetitive injuries.

But as far as Upping your effort, or increasing your degree of difficulty...don't worry...aging will take care of that for you.