Get involved in cycling communities and you realise that this, while not particularly easy, is not particularly hard either. Especially for spryer-than-average 70-year-old.
An average unfit person of almost any age, can, within a couple of weeks of regular riding, achieve 100 km in a day.
Woooh. An unfit 55 - 70 year old isn't going to do 100km's after a couple of weeks. They'll blister and cramp out at a minimum even if it's really flat terrain. However, they could get there in a couple of months with the right training/dedication.
An average unfit person of almost any age, can, within a couple of weeks of regular riding, achieve 100 km in a day.