I could have written exactly the same as Nanabilly, you're not in our syndicate are you?
You certainly need a certain level of fitness although I'm not particularly fit. Locks can be quite hard work but they are also sociable places where you chat to other boaters while the locks are filling/emptying. It doesn't take long to get the basics of steering a narrowboat so that you can manage it. Modern boats are warm and comfortable although they are ....narrow.
Many go from pub to pub to eat, we tend to cook simple dishes on the boat or barbecue when moored up for the evening. Coffee on a late autumn day is in an insulated mug with a little dash of whiskey to keep out the cold while going along. The scenery comes to you, you just sit there and enjoy it. Even towns and villages you know well look different from the canals that run through them.
You could hire a boat for just a few days to see how you like it. If you haven't been on a narrowboat before, it can seem like hard work if there are just two of you (DH and I are used to it) but I think an extra 'hand' or two would make life easier. Take suitable clothing, including wet weather gear and, most importantly, enjoy it.