If you are fond of doing exercises, you may learn kegel exercises, kundlini yoga, or Taoist yoga. You may visit anmolmehta.com. you may search for satkriya. These are long term treatment but quite effective. Your wife can participate in this programme.
There is one technique which may work is Mehan Snan –Penis Bath. This is hydro therapy. Here you stretch penis fore-skin forward with your thumb and index finger and pour water on the tip of your penis( not the whole penis) Fore skin is the junction of many nerve endings. You may pour water for 15 minutes per day. This will help in increasing sexual energy.
If the fore-skin is removed, you can pat perineum (muscle between anus and scrotum) with a cloth dripping with water. Female can use this method. Any exercise which build up pelvic floor, lower abdomen, and the are surrounding genitals will be good for sexual problems.
Welcome to 50+ ! :-) If your wife is satisfied, then you are a lucky man! Stop working out so much and concentrate on pleasing your wife - you don't say how old she is, but if you used to have sex with her 3-4 times per week, and she is getting close to 50, then you better get ready, because that is when the womens desire level begins to peak! Also as shyteen has pointed out, sleepp is important - do you have a stressful job? is that affecting your sleep at all? There are lots of different things that can affect your sexual desire, and stress is #1
Stress is the silent killer! unload the stressful things in your life, and your sex drive and both you and your partners pleasure will increase dramatically!