The 400 meter question

Why can't I run 400 meters as fast as I could when I was 30 (54.5)?

If I try to run 400 meters fast now, I often strain my hip or hamstring. So for comparison purposes, I'll use my time up Signal Hill for which my ancient PR is 57 seconds. I ran Signal Hill tonight for the first time in a long time. I thought I was flying. My time was 1:33.94. That's 64% slower than I used to be. In contrast, my best for 2 miles is 11:01 (as a high school senior). A few years back, I ran a 5k in 19:20 which works out to 6:14 pace. That's only 13% slower than 11:01 and at a longer distance.

Speed is a product of turnover (strides per minute) and stride length.

Turnover

It used to take 50 steps for me to cover 100 meters in approximately 12.5 seconds (I think I might have got this down to 11.8 seconds). 50 steps in 12.5 seconds yields a turnover of 240 steps/minute. My PR for 400 meters was 54.5. I also had several 54.9 times.
Assuming that I had the same stride length as in the 100, a 55 second 400 works out to a turnover of 218. I recently timed my turnover at 196 going uphill on an uneven dirt trail at twilight. So I think turnover is still in the same ballpark, at least within 10%.

Stride Length

Stride length depends on two things: power and flexibility. I think
these two are what really limit my current 400m speed.

1. I don't do speedwork that often nor do I lift.
   
2. When I do, I prefer this to be slower than 7:00 minute pace because a faster pace gives me an upset stomach after about 5 miles of it and
   
3. Even if I do run faster, I don't like running less than 800m when running intervals because of a tendency to strain my hamstrings or hips.
   

Which brings me to flexibility. I do stretch but my stretch for hips and hamstrings is something like touching my toes. When I ran 400m, I'd do hurdle stretches and was able to get my chin to touch the bottom of my kneecap. I'm absolutely nowhere near that now.

Does this affect performance in ultras?

Lydiard training can be a lot like ultras yet Lydiard looked at long runs as a way to train for speed. Is it possible that the reverse is true? If one practices recruiting all of the muscle fibers at once (running uphill all out), will that build strength which can be used in an ultra? For the element of flexibility, a longer stride and strength to support that would obviously improve one's time in an ultra. So I'll assume that it is a good idea to go in pursuit of my lost youth and try to improve my 400m or short uphill time by increasing flexibility and power.

Workout changes

1. I'm willing to spend once a week (Monday night) running fast uphill sprints on Signal Hill. Back in the day, I could only manage one or two of these per workout and I could only manage one tonight.
2. Start working at hurdle stretches. These are the most painful stretch I can remember (probably why I haven't done them for years). Also, I remember that I used to cheat at them by letting the fore leg roll out so I'll watch that.
   
3. In addition, I probably need to do some hip stretches. Since my schedule is tight on Tue and Thu, I'll plan to do these on MWF and weekends.