how old are you ? What health do you have ?
Dave - I am in awe of you cracking on with your cycling with the health challenges to which you've alluded. I hope I'm cycling at your age. At about 6:30am on Sunday I rode alongside another 70+ year old at the 380km point of the BCM 600 audax, who then caught me at the top of a hill which took more than 45 minutes to climb (in North Wales). I was massively impressed with him, despite senior grumbling about how age was eroding his power and make climbing harder. He had water in his bottles and a 32t largest sprocket.
I do possess a Senior Railcard and, compared to you it seems, have had minimal health problems (touch wood).
But I don't see why you think (imply) that eating proper food is a poorer option than powdered stuff. And if I was going to take powdered stuff on a ride like the one you described, I would stick with the powder you know and which agrees with you/your gut, and not go out and get some random sachets of powder.
Remember that the makers/sellers of powdered stuff and 'energy drinks' have a strong commercial interest in encouraging people to think that such hydration/nutrition is vital to a rider's performance, however short the ride.
As far as
@S-Express is concerned, be aware that he can ride with his heart rate at 170bpm for more than 3 hours, no problems (he has said) so I don't think he has a Senior Railcard. But his advice (above) is still good, even if he's really young.