Be very careful when adding in sprints. Your feet ain't ready for all the new torque your legs can provide now. Break into them in the 80% intensity range or something. Last time I picked up sprinting again around the 3rd time I was out I messed something up in my foot that had me in a boot for 9 months. It definitely was not worth it so if your feet are already feeling beat up just remember they are far more out of shape than your legs and glutes powering them.
Secondly, like you said sprints take a lot out of you, and are really going to hit your recovery ability so if you are adding in volume via sprints you need to be cognoscente of that and remove some volume from your leg training if you want to keep making progress in both areas.
Kinda funny you say that. My wife and I were talking about cardio and she's like, why not throw in sprints? I said because that's going to open me up to a whole slew of potential injuries. She didn't agree with me, but she's tiny and built like a little runner sorta lol.
Since yesterday was my first effort throwing HIIT back in I did the long warm up on an incline then i think it ended up being 6-8 sprints over 15 minutes. 30-60 seconds on, 45-60 seconds off. I ran at speeds of like 6mph, 6, 6.5, 7, 7, 7.5, 8. Pretty sure I only hit 8 on the last lap.
10mph on a treadmill is a reasonable sprint for me but I have to be at a minimum of 6 to be able to open my gait at all. Today obviously was only like 5 interevals of 1:1 (min on, min off) and it was I think 6, 6.5, 7, 7.5, 7.5)
In addition to watching how my feet, calves, achilles, hamstring, and psoas responds (LOL) I was also watching my heart rate to see how high it got on high efforts and how fast it came down. The theory of course being that my max would be around 180bpm (220 - 40). whether anyone takes it serious or not it gives me a measurable baseline to work with.
I usually hit about 170 on the max effort sprints and would try to relax enough to get down to 120s before picking it up again. I want to watch that and see how fast I can recovery, and how my max speed / bpm ratio change with each other over time.
I also want to get my resting heart rate down. last time I checked was june around 60bpm I think (?) but that's much higher than when I was an xfit competitor. lowest I remember seeing was like 47bpm in the ER haha