Finding the discipline to work out?

Here is my story: I have had varying degrees of lethargy over the years. The reasons, real and imagined are just as varied. Much of mine was just laziness or unwillingness to do anything. My lethargy was caused by deteriorating joints most notably my progressively bad hips. My lethargy might well have caused my hips to go so bad. Either way, late last year, I decided I had enough of my pain and lack of mobility and by extension lethargy, so I sought help. I haven't spun my story this way but it applies I think. My surgeon invested alot of his life, time and money studying, learning, training and becoming highly skilled at performing hip replacement surgeries to get me a new chance for a better life. I owe it to him (as well as myself) to bear down in rehab and PT to get better and more mobile (Done!) and to make the most the work he did on me (continuing). My discipline lies in the fact that after so, so long I CAN and the fear, real or unfounded that if I don't I will wind up back where I was a year ago and I have no interest in that. So I go several days a week to fitness center to walk, pool exercise (my PT relocated) and walking in pool. Eventually, I might add some things like swimming.