It is a little unusual to do HIIT to build endurance for the soccer field. I would think running longer distances (maybe 5-7km) at 70% effort would be better suited. To add some aerobic-anerobic balance, you could do the first 30% distance at 80% effort, drop to 40% effort for the next 30% distance, and finish the last leg at 80% again.

HIIT is favoured by those with less time, who want to burn more calories. I would think you'd like to avoid that, as you are conscious about not losing any muscle. HIIT done properly is supposed to keep burning well after the workout too. If you must do HIIT, then reduce the high intensity bit to 30-40s, replace the 'rest' phase with light jogging for 2-2.5 minutes and don't do this till exhaustion; set a number of cycles (4 to start with), which you can progressively increase, so that endurance increases somewhat.

It is a little unusual to do HIIT to build endurance for the soccer field. I would think running longer distances (maybe 5-7km) at 70% effort would be better suited. To add some aerobic-anerobic balance, you could do the first 20% distance at ~100% effort (after sufficient warm-up, of course), drop to 40% effort for the next 30% distance, and finish the last leg at 80% effort.

HIIT is favoured by those with less time, who want to burn more calories. I would think you'd like to avoid that, as you are conscious about not losing any muscle. HIIT done properly is supposed to keep burning well after the workout too. If you must do HIIT, then reduce the high intensity bit to 30-40s, replace the 'rest' phase with light jogging for 2-2.5 minutes and don't do this till exhaustion; set a number of cycles (4 to start with), which you can progressively increase, so that endurance increases somewhat while speed increases more.