Ok this is the Steph plan.
Run7.5km would take me around 48m with heart rate in the 120s
Then walk until timer hits 1 hour That would be about another 1.1km so 8.6 in the hour, then just keep repeating.
when the watch beeps for the hour, see how far you have gone and gently run another 7.5km then walk to hour beep.
if possible do most of the drinking and eating in the walking section.
Take 10-15m breaks at pit-stops
Repeat as much as possible.