Related to this subject, you can easily divide your 31 minutes outbound into 4, 5 or 6 equal chunks and divide your 29 minutes inbound into the same number of chunks.
When you run out, at every interval drop a "mental distance marker" (i.e. remember where you are). On your way back at every new interval you can measure your progress.
Thing is on our route, which SEEMS predominantly uphill going out and down coming back you gain anyway (probably due to the section passing the Bankhouse), so you should have no problem making up the 2 minutes, though it always seems hard work on that last stretch.