I don't think it is ever too late, as long as you are sensible. I was 70 when I started to do really vigorous, long, steep walks. At first, I would get a bit breathless on the very steep climbs, but that might have been due to the altitude, as my flat stands at over 3,000 feet. Now, I find climbs that left me really exhausted, heart thumping, knees aching, are quite easy.
I do think walking and swimming are the best exercises as they don't put any pressure on your joints. Walking is free, needs no equipment other than boots and poles, can be done alone at any time and is immensely enjoyable.
I think you live in a lovely part of England, Alison, so I am sure you would really love exploring it on foot.
It is very hot here, so tomorrow I am going 'wild swimming' - i.e. in Lake Geneva at Evian les Bains. I hate municipal swimming baths.