Hi Owdcodger, I'm a few years older than you but I walk up Snowdon a few times a year. It's not easy but providing you take your time it can be an enjoyable walk.
The easiest way and one of the most picturesque is on the PYG Track. It starts at Pen Y Pass (altitude about 1100 feet) so there is only an ascent of 2500 feet!
I would estimate your walking time up and down to be 5 to 6 hours. If you do it though, pick a good day because the mountain weather can change very quickly.