No, I didn't cheat and do this one on horseback! The story about this run is that it made me realise how similar running is to riding a horse. In both cases you have to keep your body aligned so you need the minimum effort to do what you want to do - as horsey people, "tall above the saddle and tall below". Replace "saddle" with "hips" and much the same applies to a runner.
This was made even clearer by the fact that I have been feeling a bit of heat and discomfort in my thighs after the last run. I went on t' internet and found a site on "chi running" (http://www.chirunning.com/what-is-chirunning/), which gives pointers on how to align your body so you can do more with less effort. The guy who writes this makes a big deal about "core stability". Again, this fits with what I know from riding - most of what you do comes from your core, and your arms and legs are secondary to the process.
Worried about my aching legs, I gave this alignment thingy a go. I noticed that I had to keep making micro-adjustments to my posture every minute or so, but it did seem to help me keep going right to the end of the 28 minutes. And my legs actually feel better afterwards.
(The picture is of me on my friend's pony, to show what I mean.)