I did a very similar JY South African tour in 2014, when l was a similar age to yourself. I'm averagely fit for my age, and didn't find it unduly strenuous. I'd say it's a great tour to start out. Great scenery around the Cape and garden route, Kruger Park for the safari drives, and if you do the add on, Victoria Falls, with a chance to spend a day on safari in Botswana. I hope have been reassured by the comments regarding age, and book this amazing experience