Thanks prizewriter - the a,b and c not too critical although obviously I would not go to excesses in either b or c.
Luckily i'm next to a major town which has most any art you can name and I can do it. Quite spoilt really.

I most definitely won't be embarking on Muay Thai! They trained that at my kickbox club and it's just insanely hard on the bod! My kickboxing training was already tough and that was when I was at the peak of my physical fitness!

I would like to do Iai-do or Iai-jutsu (the interest still remains from my Kendo days) but there won't be much workout value I suppose.
Taekwondo is an option and an interest, but I would have to take it easy as it can pretty full contact.
Maybe Aiki-do would be a sort of middle ground...

Iaido you can practice alone and anywhere there is a decent floor and a high enough ceiling.
Aikido needs a partner and a mat. I have seen people who started after 65 as beginners, enjoy themselves at it for years.

Superbully:
I started doing tang-soo do 4 months ago at the age of 44, and mixing in some groundwork as well.
I lifted weights for some time before, but did practically no aerobics.
The biggest obstacle for me is flexibility, which I've been working on by stretching, and aerobic capacity, which has improved greatly.
I feel much better since I got into this, and dropped a lot of weight in a short time. My only regret is that I didn't do it sooner.
My advice to you would be to go for it. You're not getting any younger, and the longer you wait, the harder it will be.
I found that I like the contact part of my style, and that I learn better that way as opposed to practicing moves without opposition.
I've also found that my instructors and fellow students respect me for having the balls to get into it despite my age, and they go out of their way to help me.
You won't be able to keep up with the younger guys right away, but you'll catch up before long.

The great thing about martial arts is its variety of arts to accomodate anyone

I would defintely check out Aikido then. Aikido uses quite a lot of sword (bokken) work as well as the empty hand element of the system. Depending on the class, it can be quite vigourous. At worst Aikido will provide a moderate workout.

Some styles are more active than others. Tomiki/Shodokan Aikido uses resistant randori, so you can get a decent sweat up in class without getting half-killed!

Really though you will get most benefit out of doing something you enjoy (as you are more likely to keep at it). So I would check out as many classes as you can and see what you enjoy the most.

I did Aikido myself for a few years, so if you have any classes in mind then I can give you a (barely) educated opinion on them.

It is easier when you are younger but I don't let age dictate if I should train or not. I believe you can learn at any age but you will have to work at it. Flexibility shouldn't be something to worry about, you eventually get there through training.

I gotta say, I wouldn't give up so quickly on Muay Thai. I am 43 and do it 3 nights a week. I got more beat up doing Kenpo, but I believe it has to do more with the culture of the school. I am no longer competing and just there for the pure fun of hitting stuff (and people) and keeping my fitness up as high as I can. We have a great culture that blends the hard core competitive guys and the casual guys like me. While we work hard, we also work hard to make sure we can all go to work in the morning as well.

Personally, I think a body pushed (in the right way) says younger than a body that is babied to much.

A good school with good people and instructors will make sure you are training at the level that is right for you, and trusting you to know your body and when maybe something is too much for you to do to avoid injury.

Not knocking any other choices, just saying don't shy away from the hard stuff. It can be done and done well without injury in the right atmosphere.

I don't understand why anyone would believe Aikido is easy or made for age persons. When I had briefly trained, it was anything "but". The falls, throws, and pain from certain tactics were not for light duty

That's a fair point, but the key is finding the right school. There's a gent online called the Wise Grappler, and he has a survival guide to grappling for "older guys". He started BJJ when he was 32, got his black belt 10 years later but learnt some painful lessons along the way. He had to train with people who were less than understanding about the fact that he was a bit older than he was.

Finding a good school that accomdates all types of students is key.

Aikido training can be tough, but to be honest it is rarely as tough as a Judo or Boxing class. Additionally, the OP expressed an interested in Japanese Sword systems, so Aikido would fit the bill.

I attended a formal Aikido "testing" a couple weeks ago.
They had the top two blue belts one on one taking turns as uke.
Talk about vigorous!
@ fifteen busy minutes later they were still at it full bore.
They both passed.