We all want to be fit. Feel fit. Move in a fit way. Enjoy being fit. But the road to fitness has a lot of bumps, twists, turns, obstacles, frustrations, heartbreak and more. It is far from linear, or direct. If you've been fit in the past and are returning to fitness, it can be even more of a challenge since you know what it's like to be fit. Ideally, you would just fast-track your way (back) to where you were, and leap frog the bad parts.
There is a really important and valuable concept to understand - and respect - which is the difference between your "date fitness" and "goal fitness". Your "date fitness" is your level of fitness on the current date, while "goal fitness" should be obvious. The thing is, you should swallow your pride and train at your current "date fitness", whatever that is. If your rehab program says to run for 30 seconds, walk for 2 minutes, that is what is means. Don't go and run your "usual" 5km. Similarly, don't run at the pace you used to run at; your fitness has changed since then and that old pace isn't going to be the 'right' pace now. But it will be if you stick with your training so that your "date fitness" (or "date pace") equals your "goal fitness" (or pace).
This is also important as you build fitness, perhaps alongside others, where it is easy to feel intimidated, slow or maybe weak. Don't be. You need to be like you, not them. You are on your path to fitness, and your rate of improvement can be boundless. Train now so that you can train again tomorrow, and the next day, and the next. Be patient, and train for your fitness now so that it move closer towards your goal fitness.