G'Day from Aus, I have just got back in the saddle after about 15 years and am training for a handful of big rides, is my 53/39 12/23 combo at my age of 53 going to get me up some serious hills without getting off or should I change?

Welcome Peter.
Your 53/39 12-23 will do just fine once you get your legs and lungs back into shape for cycling.
I'm 66 (there are many others here older than me) and my main ride has a 53/39 and 11-23 setup and I haven't met a hill yet that I can climb. Been riding for 11 years.
Stop by the 50+ forum and say hello to the other old timers.

Welcome to BF. I'll let the more technically minded answer your question. I live in very flat Florida so just about any gear combo works for me.

I had an idea that it's about time I replaced my chain anyway, and if doing that I may as well change the cassette too. I thought that if I went to a 13/28 combo for a while, it might help get my fitness back up whilst still getting me up the hills, and keeping my cadence up. Then when comfortably fitter I could change back? I don't want it to be that hard that I give it up!

Just a thought?

Sounds like a good plan.

I thought that

Welcome to BikeFourms.

- and have some good rides

I see from Google Maps you have 400 meter (1500 foot) high hills right outside of Adelaide. Street View shows some great roads up in the hills.

The 13/28 is what you need for those hills.

I have 100 meter (325 foot) hills in my area, many with 5-8% grades, a few over 10% grade. I also have a "compact crank" which is 50/34, so my lowest gears (34-29) are even lower than your 39-28 would be. I'll use my lowest gears even on a 5-6% grade, so I can keep a fast cadence. I rarely miss having a 12 or 11 cog.

Your current 53/39 with 12-23 is perfect for riding fast on flat roads. And it's designed for strong riders that are very fast even when riding uphill.

Well, it's been a while since last I posted, and there is a bit to update. Firstly, the old Dedacciai which I was riding when I first posted, has taken a back seat to my new bike the wife bought me. It is a Pinarello Opera, with Campag Record group set with a 53/39 and 12/25, and Mavic Ksyrium wheel set, at just 7.6kg ready to ride, I haven't found a hill yet that I couldn't conquer.

I have riden many hills on the new bike, for those of you whom are farmilliar with Adelaide, they include, Checkers Hill, Corkscrew Rd, Old Willunga Hill, Parawa Rd, Menglers Hill, Hill Rd Kersbrook, and my favorite, Anstey's Hill. Man what a difference a few K's makes.

Since my last post I have clocked up over 1000km in organised rides, from 80 - 250 km a ride. And with about 2500km in training, I am well on the road to fitness, and loving it.

Sorry, I'll stop rambling on now.

Cheers all and ride safe