A couple of things to try:
- when you finish a stroke you are in about the right position, but you get the next bits in the wrong order. Let your hands go first while you're still leaning back, and get them as far away as you can. Then lean over, pivoting from the hips until you can feel the stretch in your hamstrings. Keep your back fairly straight, dont go *too* far, but you need to get the handle ahead of your knees and only then you bend your knees and start to slide forwards.
- it's the same thing but in reverse during the stroke... again you're in about the right position at the catch but you're doing too much with your back and arms before your legs are finished, so concentrate on keeping your shoulders forwards while your legs do most of their bit and only then start to lean back and bend your arms.
try dropping your feet a couple of holes on the footplate and do five minutes with your feet not strapped in, that should bring you into a better position at the finish (take the first few strokes gently, and point your toes the whole time!)
watch this:
https://www.youtube.com/watch?v=yVkMzEXChLc (she's exceptionally flexible, not many people can rotate forward that far!)
For 20 minutes, try and get the rate down to 22/24ish. This will need moar power from your legs per stroke, but if you get the movements in the right order it should be easier to get there.
