Flying to Australia /New Zealand

Personally I don’t recommend a stop over I go to NZ every few years and just find that a stop over doesn’t refresh me but makes it all a longer experience
It is a heck of a long journey, door to door to NZ including getting to and from the airport is usually 31 hours for me and the only terrible part is the boredom, not so bad if you are flying with others, up until last trip I d done it alone.
I watch the films, talk to everyone that listens and follow the crowd when stopping off anywhere
I wore the compression stockings part way and had a toilet visit every few hours I have a wash and brush up in the loo and the water is fine I never thought about bottle water cos you don’t drink it when cleaning teeth or washing face etc
I rarely sleep, apart from catnaps wish I could that would pass the time better
If you’re travelling with the friends as apart from meeting them there you ll be fine you ll have a blast and the time will pass fast with chats and giggles
Good luck

A couple of days stop over in Hong Kong and/or Singapore may be an idea if you can afford the time and money. Both are wonderfully interesting. I met a lady en route who always stayed a couple of nights in Singapore airport to break the trip. It has everything! In Hong Kong we stayed at the BP International Hotel. We thought it might have something to do with the petrol industry. But it stood for Baden Powell and there’s an enormous portrait of him in the marbled, chandeliered foyer! Very smart scouts trotting about!

Say yes to those ugly stockings and comfortable pants/outfit and it is recommended that you move about/stand/walk every hour although I think that is excessive. Get an aisle seat so you don't have to crawl over anyone. My hubby and I take aisle seats opposite each other rather than one of us be in the dreaded middle seat. You can even just stay where you are and stand which makes a great difference.
I always take a couple of longer visits to the WC and do things like brush my teeth (use bottled water never drink on flight water) and a little face cloth wash about. Make a big difference to ones outlook on landing too.
I have invested in a head rest that looks like a neck collar and ends the fear of mouth agape snoring and head snap.
You can do it!!!

Eglantine's advice about compression stockings and footrest is spot on. I would only add that you should keep well hydrated and take advantage of the free leg massage chairs at the airport if your stopover is in Singapore.

I went to New Zealand last October and I’m only a bit younger than you.

I only had an hours stop over in Hong Kong and just kept walking round the airport to keep the circulation going.
Coming back I stayed for three days as part of the trip.

I wore compression stocking for part of the journey and took an inflatable footrest which raised my feet.

Just get up and down and walk about a lot on the plane. It’s a bit boring but don’t miss this great opportunity ?