You are either humble about your speed or unaware of how fast you are. Tell me how many people at your age can run 50 seconds in the 400m? I bet you ran 48 or 47 seconds in your early 30's, back when you ran 1:48 in the 800m. Seriously, Jman, you are very talented compared to the average person; and your speed is way faster than 95% of us could dream about. I was on a team of 120 male track & field runners, at the University of Wisconsin - La Crosse. For many years we were national champions in track & field. In the 400m training group there was anywhere 20-30 runners (more like 30, typically), and only about 25% of those guys ran below 50 seconds. They trained hard, seriously hard on both speed and speed-endurance. People like Andrew Rock, who was a world class 400m runner, came from that group, so you know they didn't just jog around. I recall watching our top 400m guys do 8 x 400m in 56 seconds down to 52 seconds with a 2 minute walk recovery, and I saw the top four guys run 10 x 200m in 26 seconds with a 2 minute recovery. I saw fast 50's, 100's and 150's (all-out). I saw them run repeat hills at balls-out effort. They were not sloughing off, I can assure you, and those guys were running workouts all fall, too. The ran from September to the end of May, doing hard reps, hard weight training, hard everthing, and lots of drills and hills, too. Again, only 25% of those guys ran below 50 flat. Now, here's the point, the 30 guys who trained for the 400m had been sprinters in high school. They were NOT distance guys dropping down to the 400m. They had trained seriously for sprints, typically, for 4 or more years, yet only 6-8 guys per year could break 50 seconds in the 400m.
Personally, I could have trained for 10 years, doing 400m training, and never hit 50 flat. I did a ton of speedwork in high school and a lot of speed-endurance workouts as a middle distance guy in college, and I never broke 52 seconds. I could, however, run 56-57 seconds, walk 2- minutes, and do 4 of those. Yeah, that may seem like an easy workout for you, but I was running just 3-4 seconds slower than all-out. I was tested in our Human Performance lab, when I was a graduate student in Exercise Science, for fiber typing, and the results showed 72% slow twitch and 28% fast twitch; of which 7% was type b (now called type X) - the most explosive kind. I didn't have the natural fiber types to run a fast 400m or fast 800m. I was a typical 5k runner, in terms of fiber typing. The only problem I had was lower leg compartment syndrome, which prevented me from running enough mileage to use my natural talents.
It doesn't matter how much speedwork you give me, I am not going to run 50 flat! Sorry!. It's like saying Albrerto Salazar didn't do speedwork, and he was sloughing off, because he couldn't break 55 seconds in the 400m. I've briefly talked with Salazar, in Portland, OR at track clinic at the Radison hotel where he was a guest speaker. He said he could run 16 x 200m in 30 seconds with a 30 seconds rest, but he couldn't run under 55 seconds to save his life. He said, "I was strong, very strong, and I had great mental toughness." He just didn't have the fiber types to run a 400m fast. He was a 93% slow twitcher, according to tests performed on him. NO amount of speedwork emphasis in the world would make Salazar a 50 flat runner. Sorry, aint gonna happen! Salazar even went to Houston, Texas to be trained by Tom Tellez, the world-famous sprint coach, and Tellez told him the same thing; that Salazar was never going to be a fast sprinter, but Salazar could improve his efficiency of sprinting to carry his limited speed further (this was from Salazar's mouth!).