Thanks everybody; I agree with everything stated.
To add my two cents, I just reminded myself how much better sprints on grass and running up hills is compared to concrete or treadmill cardio. Never again will I do either of those.
But, if you're built like a lot of us are, there's a good chance of you falling on your ass or twisting something.
SO, I just bought sprinting cleats with metal spikes (replaceable so I can change lengths) and some rugby boots (think heavy cleats with wide fat spikes).
This makes running on grass and dirt much better, more fun, and safer for me.
I only do sprints once a week, and long slow empty stomach cardio on the weekends. I am not doing it for health reasons, energy, and because I feel like a walking bag of toxic shit.
Truly Blessed.