While you're reading take a look at McMillanRunning.com , it's a good site with the REASONS why to run different distances/paces... and rest. Lots of runners run less than 20, seems that those measuring in Kms tend to find another 'round' number which is closer to 18 miles. Miles per week also makes a huge difference and one line of thought is that you should not be running more than 25% for your long run so if you're not doing at least 80 mpw then running 20 will over-stress your body. I think 26 is too much for a training run as the recovery time is too long, especially for a runner 'past prime'. I'm 58,been running for close to 25 years.
Find a program, like Hal Higdon (you might try Intermediate) or Pfitzinger. Count back from your race and see how soon you can get into the pattern suggested. A longer run every three weeks is great but 19 and 20 are just about the same, so you're doing two longer runs every three weeks. Lost of programs alternate longer with long (20 then 16). But we are all an experiment of one. Post back with results, that's the best part.