I'm an old guy (68 yr) and I take about 9-11 SPL in a 25 yd pool, so you are way ahead of me. I think that there's a lot of room for improvement in my stroke though, work in progress. No way I can keep up fly for longer periods of time without rest pauses in between. I have trouble just with 50 yd, but I am hoping to get better at this. I need to get myself more relaxed after my arms go in up front but before I start my next arm stroke and that will help. I'm also trying to gauge the kick I use during my arm stroke to be just enough to maintain a horizontal body position during the stroke. If I can get that down better, things should get easier.
Did you swim fly as a young person competitively?