I wanted to chime in on returning to running. I am a decently successful runner, able to run several half marathons a year, none of them fast, all comfortable.
I followed a C25K program at six months post op, the one Arroyo, who has enjoyed a highly successful return to running, posted. What I did those first few weeks did not look much like running. I didn't wear my Garmin because I didn't want to know how slowly I was moving. Three weeks in I had to take time off because my right hip was not quite ready to advance. The break did me well and I was able to pick up where I left off. Later, the schedule was too aggressive for me again. I was able to stick with it, but then had to reduce my volume and recover. At one year post op, I ran a 10K in a little over an hour. A year later I ran a half marathon just to finish. Since that time I have been able to run three halfs in a six month time period (I live in South Texas where it is hot, so I run long only during the cool months of the year!), but only get my 5k time down to under 30 minutes and my 10k time down to just under an hour. My half time is hovering just under 2:30.
Running feels great. I recover quickly. I have been proud of being able to beat some of my kids' young friends in longer events because I don't go out too fast and collapse. So why can't I get faster?
1) I got deconditioned. The last time I ran fast with ease was almost ten years ago when I was 48. Back then I could run sub 24 5k and around 4:00 marathons. At 50 yo, I was running 2 miles with difficulty. I had surgery at 53. What little speed I had was earned thru years of consistent running. I lost it during those years of being in pain.
2) I got busy with other things. Pre op I spent time fantasizing about running and thinking about how much I missed it. But I also got on with my life. I traveled, I started a new job, I went back to school. I filled the time I used to spend training with other things. And I am still doing those things today. I don't have the focus I used to have. I have said "now I will start running fast again!" and begun running intervals or tempo runs and worked on increasing my speed, but then I'll go out of town and stop doing it.
3) I got older. My hips are in great shape, but my knees, my back, my other parts are older and more beat up. When I train consistently I improve, but as soon as I slack off, those gains are gone!
I still hope to get faster, but it isn't the priority it used to be.