I’m also in my 40s and have been building my muscle over the last year having noticed a decline in tone. I worked with a personal trainer for a bit to get confident using free weights in the gym and to help judge who on the internet has good form. I did that for six months but now just do it myself and use online tutorials, as being in a gym all the time doesn’t fit with kids and a full time job!
I now use an app called “workout” (the logo is black and yellow) that is excellent if you want to put together weights routines to strengthen your body. You can either follow their set programmes (but these might not suit your life style), or you can put together your own programme—they make it really easy for you to choose which muscles groups you target and to pick exercises that work with the equipment you have access to. That’s what I’ve done - lots of dumbbell exercises that I can do at home. I’ve put together 3 or 4 routines I can do on different days, that alternated upper and lower body. I aim to do 3 or 4 exercises (each with 3 sets) in each workout, and try to progressively increase the weight in each set. I use the free app, but there’s a paid version too.
On instagram, the person I’ve found really useful for weights tutorials is Miriam Fried. She breaks down stuff like how to do a Romanian Deadlift with dumbbells into really easy steps. If you can learn how to squat and do hip hinges well, and how to brace, that helps with a lot of weight-based exercises.
Lastly, I’m also fan of Tyrone Brennand (Be the Fittest). I have the annual plan which is £120 per year and gives you access to a really good selection of strength, fitness and flexibility work outs—lots of HITT, but also yoga, weights etc.
If you want to build muscle, you need to make sure you’re eating enough protein. I wasn’t, but I used My Fitness Pal for a while (pro version) to track my macros and make sure I’m getting enough. I aim for 40% protein, 40% carbs, 20% fat. I wasn’t eating anywhere near enough (I’m veggie)! I’ve added more Greek yoghurt, soy/quorn, protein yoghurt, eggs and protein powder into my diet and now I have 100-130g protein per day.