I have just spent half the day relaying a patio, sieving shingle and messing around in the garden, I find the following help:
1 - I am not as young or as strong as I used to be, I accept this, so when I am doing something physical like patio laying, I do perhaps 15 minutes or half an hour and then stop before I feel I need to take a breather, make a coffee, check out the internet, and half an hour later I return to the physical work. I find this way I can work all day, because I am actually only doing it half the time.
2 - There was something I read a few months ago about there being a particular chemical which prevents muscle degradation in the elderly, I can't think what it is called, but it is found in abundance just under the skin of apples, so I aim to eat an unpeeled apple every day.
3 - It was just about warm enough today, out of the wind, to work in the garden without a shirt, so right at the moment I am feeling fine, my Vitamin D levels topped up by a little sunshine.