How do I increase my sprint distance?

So... I've incorporated butterfly sprints into my workout. When I first started, about 3 months ago, I could do one 25yd fly sprint about every 3 minutes (because it took me that long to recover). Later on, I was able to do five intervals of 25yd of fly sprint + 25yd back kick (relaxed), with a 60 second rest in between each interval. Now, I can do ten intervals of 25yd fly sprint + 25yd back kick (relaxed) without resting in between each interval. My long-term goal is to increase my fly sprint distance to 100yd per interval (I haven't decided on how many intervals yet). My short-term goal is to increase it to 50yd per interval. My problem is that I still can't break 25yd. Twenty-five yards of fly sprint kills me every time. The only difference seems to be in the amount of time it takes me to recover from each interval and begin the next. Anyone have any suggestions? I would greatly appreciate them. I know I'm doing something right because I can do more intervals without resting than I could before but I just can't figure out why I'm still near death at the end of each 25yd sprint.