Well, I don't know what happened, but I went way past that landmark tree this morning before the app told me I was half way; turned around, flagged a bit towards the last 5 minutes and pushed on after I was told to start the cool down walk. It was only for an extra 30 seconds to make it to a signpost, but I was determined to get almost to my starting point of the running section. 3.91km in 41.21mins including the walking sections. I'm a sweating, red faced mess at the end of it, but I can't believe I'm running consistently for 28+ minutes!
That 5km distance doesn't seem so far out of reach now.