To be really worried about her mobility?

My mum (60s) had similar issues. Major iron deficiency. Easily fixed, but impossible to get better without treatment.

Absolutely not normal.
I think fitness tapers off more through ill health in 70s. Unless something significant happens before hand.
I have one parent 70 something cant walk to the end of the street, mostly self caused, cant walk to the front of the street.
The other walks 5 miles several times a week and does pilates and over 65 aerobics.
Inlaws do park run. Bowling. Have a dog and are in a running club. Shorter distances now and being more careful on pacing etc.
I think she needs to be investigated as this is far too young. People are working full time till age 68.

My daughter really struggled walking for long when her iron was very low. She got tired very easily and laughing for too long made her breathless. She is 20 and very healthy otherwise! Worth looking into.

It seems like the heat is an issue if she is not like this at home. How is her hydration? If she is not drinking enough that will be fatiguing her.

It probably is worth seeing her GP though.

What about posting on the Travel section here, asking for suggestions for an itinerary for the day trip to suit someone with mobility issues? There may be some good places with lots of seating where your Mum could chill while you looked round a bit more.
Agree with other posters about getting her to GP for an MOT on return home.

It's lifestyle rather than age, unless she has a (diagnosed or undiagnosed) condition. I'm 70, I've been very unfit, in the past, much younger than her. Very sedentary lifestyle. I have, reluctently at times, made changes. I now do a 4 mile walk twice a week, with a friend who is 77. I now work in the garden whenever the weather is fit enough. Using a Fitbit and having a calorie target has helped my motivation. My inclination is still to be lazy, but I know that won't help so I make sure I make my calorie target at least 5 times a week - and enough extra on those days that my weekly average meets the target

She’s far too young to be struggling like this. Something is wrong. It may be something quite simple, but she should see a doctor. Perhaps you could try phrasing it that at her age she’s got a lot of living still to do and something is holding her back.

Interesting. I wonder if some anxiety is expressing itself physically with sore back, hips etc, or if it is the heat. If not breathless maybe not heart.. Or she could just be being difficult!

Something is definitely amiss, my parents are significantly older and would only need one or 2 breaks

Please encourage her to see a GP. I don't want to alarm you but my aunt was the same at 61 after being fit in her 50s and the exhaustion was due to cancer / low iron. That's worse case scenario! But a 60 year old with no obvious health issues would usually be walking fine. I'm 55..

Agree with above about getting checked.

For the day trip, if it's to a city then a hop on/hop off tour bus can be a good way to see the main sites without lots of walking, and can have a rest on the bus between sites.

Low iron, Vit B12 or Vit D can cause fatigue so a GP check should be the first port of call. But so does not using her muscles, is there any keep fit classes for seniors (or chair based) nearby or can she do some from YouTube at home to increase her stamina?

I have health conditions so if I'm out anywhere with family I get them to "park" me somewhere. If it's a lovely day and there's a bench I sit on that and people watch while they explore. If not I have a cup of tea in a cafe and they disappear for an hour. They take photos so I still feel part of the day out and that way I get to rest in between all the walking, so walk, rest, walk, rest, walk, rest. Would that be an option?

Yes I agree get it checked by GP.

I am a bit younger but not much and very aware that I am quickly out of breath; I was a runner for years (not fast but long – 10k/HM) but now I cannot run even a couple of miles without stopping. GP told me my vit D and folic acid were low. I am also at least 2 stone overweight due to, well, eating too much and sedentary job.

But even I can walk about the place without needing to stop every 15 minutes. I could certainly manage a day trip out. So I think your mum's mobiloity is a concern.

Please encourage her to see her gp and take from there , have a nice day out with lots of breaks for her .You'll both enjoy it more .
I'd give a lot for another day out with my mum.

My older family members are like this and it is a very sedentary lifestyle and being very unfit. I have actually become a lot more active when I’ve seen how their mobility has declined in their early 60s, with the goal being long term strength rather than weight loss.

I hope you have managed to enjoy your trip anyway!

on the fitness note, I’d echo sending her to the GP, but if nothing underlying is found, encouraging her to move more. Anything from (chair?) yoga to swimming. It’ll offer loads of benefits as I’m sure you’re aware, but most of all at the moment can protect her independence and ability to enjoy life.

I'd definitely get her to see a GP if you can, my parents are 10 years older and can easily walk around for a whole day if they're going for a day trip and regularly go for walks that take a couple of hours without stopping. Early 60s seems very young for her to feel this way.