As my ankle started to be irritated after 20km last week, it seemed wise to stay below that threshold. Today's run was 16km up to Oxstones and back. Actually I think it's harder than some routes of similar length as it is very much up and then down. This does give quite a strong training effect as the ascent is slight, but basically continuous for over 50 minutes. Then there's the eccentric loading all the way back.
As it's a slow run day I watched my heart rate closely and the chart shows that I did a good job of staying in the zone.
The good news is that my ankle is fine; this might have been helped by the change of shoes too. Time will, or might, tell.