Ryan nailed it. This 53 YO would love to trade up to your 40 YO potential! Funny, I've been chasing the sub-20 5k goal myself for the last two years – still searching for that last 57 seconds. And while you're further away your upside is a much greater as you could benefit substanially from increasing mileage / building a base and the consistency that stringing a coupe of years of solid training will bring.
I do 800m repeats on the TM too and while I'm faster than you I don't even try to approach the speed you're doing. 10.0 mph is my ceiling. I can get down to 5:20 pace for 16x200m repeats but I know better than to try that speed for 800m.
Training should be focused on your current fitness not the fitness you hope to one day achieve. By doing this you will get a greater return on your investment by maximizing the time spent training. This will make you a better athlete which in turn will put you closer to your goal. It's a process.