It's 78 weeks.
At half a pound a week, you'd have lost 39 lbs in that time - and as that represents a calorific deficit of just 250kcals a day. You could do that from upping activity levels alone or making a couple of changes in a day - such as a burger without a bun, not having a chocolate bar or packet of crisps, dropping a sugar in coffee, just small things that aren't as difficult as you'd imagine. Not drinking alcohol will cut a load of calories, as long as you don't swap them out for exactly the same quantity of sugary drinks instead.
Whilst you might be told that the only way to increase muscle is to lift heavy, at knocking on for 50, you're prime age for fucking up tendons by going too hard or finding out that previous unnoticed age related changes in your spine really hurt. Machine based weights or rowing is safer for you, as is swimming, then throw in some gentler weightbearing exercise, such as a short plod around the park once or twice a week (get a gait analysis and proper running shoes rather than £10 Primark trainers, you'll feel the benefit everywhere, not just in the gym or park). The extra strength in your leg, back and core muscles will make a huge difference to how you hold yourself, how able you are to do other exercise and how able you feel to cope with daily activity. A strong core reduces both overuse/postural issues and likelihood of accidents in the first place.
Make sure you're well hydrated, have good quality food that makes you feel happy and healthy, get as much good sleep as you can along with daylight and fresh air. A multivitamin might help, especially as most people are low on Vitamin D and the Bs.
Concentrate upon living well and living happy. Take care of your skin, teeth, feet and hair as well as challenging yourself physically and it will really show by the time of your 50th.