Thanks for your reply Skeezyks, that has given me a few things to think about.

I'm in my mid years and have wondered whether my age is a factor, certainly when I'm tired I feel every bit of my age in these old bones and muscles.

Cold weather - well it's not as cold here as your place (brrr!) but I do think cold and wind has an impact on me, roll on Spring.

Sleep is an excellent point, I do sleep okay but often wake needing the bathroom, this is not something that used to happen before. I drop off again no bother but the sleep is broken so maybe not as deep.

Muscle groups - yes, you could very well be right there. My GP actually suggested a lay off from running to rest that muscle group, but I haven't done this as so much of my life revolves around this activity. I have an event coming up so now is not a good time to rest, but once that is out of the way I'll take a week out and just swim.

Thank you so much for your thoughtful consideration, I really appreciate that Skeezyks - keep up your walking, I enjoy that too, it's a wonderful low impact exercise.