@TrainForPain here’s a long winded answer…
I was doing the same thing as you before… I would start a quest to become athletic again and then give up and go back to a bodybuilding or powerlifting routine.
I came to the realization that I had to make some changes in May when I was playing basketball with the kids and noticed that lateral movements were extremely difficult and painful… it was embarrassingly hard to even do a crossover let alone get into a defensive position or land from a jump shot. Adding any kind of exercise that would help with overall athleticism and lateral movements was my first step.
I slowwwwwly started adding new exercises to my regular bodybuilding type training (ex: jump squats, jump lunges, skipping, burpees, clapping push ups, side shuffles, lateral lunges, skater lunges, Olympic lifts etc…)
Hill sprints and stair sprints: In the beginning I couldn’t sprint so I would walk sets of stairs and each time I tried to increase my intensity. After 4 weeks I started sprinting 10% of my sets and continually increased from there.
Zigzag sprints: Once I could sprint again these were extremely helpful
HIIT: A few years ago (my powerlifting era) I went through a phase where I thought I didn’t need cardio… that came back to haunt me! I got a stepper in June and started by doing steady state cardio for 30 minutes 5 days a week. I bumped it up to 45 min after the first 2 weeks and then added interval training into the mix. I love interval training and prefer 60second intervals with 60 second breaks… I’m on my 5th month using my machine consistently and can do 45 min intervals without dying, great success!!
I’m not going to lie, it’s been a big struggle for me and I was getting extremely frustrated but I’m on the right track now and feel so much better.
Good news: We were short players last night and had no subs, therefore, I sprinted around for an hour without dying from exhaustion or getting injured 
#lifeisgood
Start by adding something enjoyable to your routine and keep it there! What sports do you like? I really enjoy being outside so mountain biking was also helpful for getting my butt moving again and just picking up a basketball and shooting around for fun. It’s certainly not going to be easy but if you can try and have fun while you’re doing it then you’re more likely to stick with it…. I know I’m telling you things that you already know.
It’s crazy to me that I’m at a point where I have to do these things in reverse. I was once a young, athletic, extremely fast human being who wanted nothing more than to learn how to eat properly and build a muscular physique. Now the new challenge is to keep a muscular physique and become a great athlete again! Can I do both? OF COURSE!!! 
