Doing this with a treadmill is honestly a gamechanger if you can't or don't want to do cardio outside (e.g., safety issues, heat, bad weather, etc.)

Even just walking on an incline can get your heart rate into the right zone so you get cardiovascular benefits while being so low in fatigue that you can get a lot of volume in.

Eventually your fitness will increase such that you'll run out of incline and speeds you can walk and you'll have to jog to stay in the zone.