You started with too much too soon. It might increase your work capacity in the long run, but for now it's affecting your recovery. If you want to increase work capacity you just need to train with higher volume than usual (to the point of overreaching) now and then. Cardio and conditioning are mostly beneficial for your heart and circulatory system. I heard that Greg Nuckols said that you should do cardio if your resting heart rate (measured first thing in the morning) is higher than 70bpm.
Also, these days a lot of experts (like Nuckols, Eric Helms, etc.) recommend low intensity steady state cardio over HIIT stuff like sled drags. It's much easier to recover from and has the same health benefits.