As somebody who is also 56 this year I can see where you are coming from. I think it maybe part of the menopause, it makes you suddenly realise that you are not 20 anymore in a very physical way.
When I hit 50 I also started putting on weight, got a bit lazy, tea and biscuits were a great comfort, ditto chocolate ditto wine ... then, last year, I looked in the mirror and thought - no way, not yet, and started walking, then jogging, then running.
I am now training for the London Marathon, have lost the weight and my doctor tells me I have the fitness level of a 30-year-old. So, within reason, anything is possible at any age and I now no longer think of my age. All I think about when I wake up in the morning is: nothing's hurting, I feel great, let's get on with it 