An 800 specialist, whether open or master, would not average more than 60mpw over a year, but he/she would likely spend as much total time in preparation as someone running 80-100mpw. At that end of things, the strength/pylo dimension becomes as crucial as the total volume run, and those sessions can be very time consuming.
To direct this specifically to masters, however, I would say that it's probably a mistake to specialize as narrowly at one event as it is necessary for an open athlete to do. If you look around, you'll see that the most of the best masters 800-1500 people, and even some sprinters, will run a full X-C season with their clubs, and may race on the roads up to 10k, and even above, especially as they move into their mid-40s and beyond (see, e,g., Ed Whitlock, who will race 800m to the marathon in one season, or the mind-blowing sprint world record holder Karla Del Grande, who runs X-C every season). I generally recommend that masters athletes attempt to be simply "distance runners", rather than narrow specialists. This is a healthier approach for the older athlete; its also therefore the approach most likely to produce greater long term consistency in the athlete's favorite event range.