LOL!!! 41 isn't old .............. still young blood
In my opinion , I would do a regular sport that you enjoy and eat right . Its all about eating right. For example , you may play football once or twice a week , and that requires a level of fitness ( so your aim would be to stay fit , stamina wise and muscle development )
do Cardio work , exercise bike , pressups , squats which require no weights .
Warming up is key
1. do jumping jacks where you spread your legs and clap your hands over your head with stretched arms. Good for warming up the full-body
2.alternate toe touches standing up , where finger tips touch alternate toes. Good for warming up back , buttock and hamstring
3. Squats , good for warming up quads,hamstring and buttock
4. pressups good for pecs.deltoids,triceps, and upper back
5. seated stretches where your aim is to touch your toes alternately good for hamstrings,groin,spine , back and buttocks
6.thigh stretch , hold foot and pull towards the bum , ( stretches thighs) good for quads and thighs
can start with doing 10 reps then increase slowly to say 20-25. Its recommended 10-15 mins warm , its very hard to do but something is better than nothing hey !!!LOL
Obviously older you get , its harder to build muscle and more prone to injury so stretching is crucial before and after training . Its hard to do and is easily missed .
If going for weights , i.e. benchpress, squats , biceps, legs etc... focus more on technique and with light weight don't go or increase weight until you can comfortably do 12-14 reps for about 6 weeks. Then increase slowly. Your body will tell you if you are straining or easily doing the weights , then act accordingly. I would advise not to lift too much heavy weights. Do compound exercises as a must i.e. squats , deadlift ..
Theres plenty of stuff on the internet giving you tips and techniques. Obviously body needs rest so train Monday , rest , train weds, rest , train fri, rest ....
I believe 80 percent is what you eat , let me add I am no fanatic or an expert just some views I thought I share and if you like them take on board otherwise just disgard them , but the biggest hurdle is starting it , whatever you want to do , start it and stick at it ....... it will take time and you'll notice the benefits .... Next time you know it you'll be 51 but feel like a 21 year old.