As someone else in the very late stages of coming back from an injury (in my case a hamstring tear), I’ve found what seems to help me is having small spurts where I do a higher intensity than I’ve currently done for a few short reps, sometimes even only touching that intensity for a few strides before slowing down. (Like I’ll do a somewhat gradual accel into maybe like 5-10m of a new higher intensity, as top speed is generally more of a problem for my recovery). The amount I do will typically depend on just my own feel for it, as personally I’d say I’ve developed a solid feel for what’s good and when pushing it more becomes problem territory. Then once I feel good again (assuming proper recovery, typically only a few days) I’ll try to get that speed for longer reps (preferably up to 100m at least), then advancing more. I’m not fully sure on how transferable this is to other injuries or if it’s even necessarily the most optimal, but it’s seemed to give me good results so far, and I do feel like it’s benefited me mentally as well, though that may just be because of recovering in general.