Hi Doctor, II Am A 68 Year Old Male, I

Hi doctor, II am a 68 year old male, I use the gym 6 days a week, included in my visits is a 30 minute boxercise class 4 times per week, the rest of the time is spent weight training, I think for my age I am very fit. I am 5` 10" tall and weigh 11 stone. I consider I eat healthily, including lots of fruit, vegetables, red meat, fish, chicken, brown rice, I also include vitamin substitutes and protein shakes I am a non drinker and do not smoke. MY problem is , when training my muscles look pumped and great but as soon as I stop all my muscles disappear and I don't look as if I have ever been inside a gym, which is very disheartening, also I have been training for about 4 and a half years in total. Thanking you for your help.....Terry