10,000 steps is a long distance and not something to just be achieved in a day.
Anyone of any age, coming to exercise for the first time, will find it a struggle. DH having had a bit of a medical shock; a diagnosis of sleep apnea and official 'advice' from the doctor not to drive until it is dealt with, is finally coming to grips with his excess weight and lack of exercise. The first time he walked the bare half mile to the newsagent and back to collect the papers, he almost collapsed from the effort. Now he can do it quite easily and is planning to gently phase in more exercise, but note the phrase gently phase in.
You cannot go from nothing to running marathons in a day. I am quite fit and active, but a couple of years ago I hurt my back and was inactive for three months. I was shocked when I resumed my usual daily walk to find, that after 2 walks on 2 days I had to take the third day off to recover. It took me over a month to reach the daily level I had taken as normal only a few months earlier.
You will need to adjust your food intake to lose those three stone, exercise alone will not be enough, but there are a whole range of diet threads on the Forum pages discussing all sorts of approaches to losing weight. It doesn't have to be a slimming club.