From what i remember, Todday suggested doing only ME all out sprints for you to teach your body learn to move fast and not worry about anything else. I think a similar thing applies to me also in that even in my maximum effort sprints on video i look to be moving in slow-motion. So by going all out every time, you'll be training speed, and nothing else. For fitness get it from running. I'm prioritising building an aerobic base before i worry about sprinting because while my lack of speed is a huge weakness, an even bigger deficit exists for my fitness. I'll get fit first then get fast and in doing so i'll get even fitter as well rather than trying to do it the other way around. Anyone can get fit, even a lot fitter, not so much can be done about how fast you are (in the sprint sense), that's probably more genetic than anything else. Right now i've trained my body to recover better by running every day. Even my hamstrings are bigger which i found surprising because i'm not doing any sprinting at all. Just daily squats + running. That seems to be working really well for me, fitness is coming and surprisingly my strength too.