Fit in my 40s: what next after the Couch to 5k app?

The best-loved in-ear coaching app for runners is Couch to 5k, which is free, NHS-supported, and excellent. It plots the minutes of each run as you build up, syncs to your music, dispenses motivation and advice, and always reminds you of the most important thing: don't stop, even if you want to. On a randomised test of me, I can tell you it works. In the absence of anyone telling me not to stop, the minute I experience even the slightest discomfort, I can think of no good reason to carry on.