The short answer is: Start slowly. Give yourself time get used to using your muscles naturally, to relaxing, and to your gait changing.
For running, you might start with a VERY short run… like 30 seconds! If that’s not fun, there are a number of hints on our site that will help you. When a short run feels easy and fun, add 10 more seconds to your next run. Then “lather, rinse, repeat.”
Listen to your body. Use fun as your guide.
This blog post may be helpful.