52 yr old male from the UK

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Where to start. I don't need to lose weight as I have lost what I need to lose. I am currently 11st 10lbs which going by all the NHS material means I am technically overweight for a man of my age at 5ft 8ins. I do not want to lose weight but want to lose body fat and to gain muscle. I am an ex-RN matelot who has been into keep fit all my life. I have recently seen a pic of myself on the beach in Barbados (navy day out) when I was 39 and realised that I need to try go get my body back to something similar. I eat healthily but I also live. I like real ale, red wine and whisk(e)y but generally only drink alcohol during the week. I appreciate I am not going to have the body I had when I was 30, or like these people on the likes of youtube who say eat that and do this training. They are all in their 20s and 30s!!! However, I do want to look as good as possible when by the pool (vanity!) and want to be able to drink beer without getting a big belly.