Damn it, re-reading your post you need to improve your flexibility in quite a few directions. As well as the above, you should practise this:
The guy in the picture is doing it really nicely. They key here is don't try to make your legs as wide as possible right away, make them a comfortable distance, like the guy in the photo. Put your hands between your feet and tuck your elbows in, and come down without bending your spine- just like with the seated forward bend.
Now here's the patented yoga magic... feel that excruiciating pain? Feel the arches of your feet lifting up? Feel your knees buckling? Feel your legs generally wanting to give up? Press your feet really flat against the floor, lock your knees and tense those thigh muscles. The pain from the stretch magically disappears and the tendons relax a little. Now you can go a little further. The pain comes back, and so you have to push and straighten and flatten your legs again.
The same principles hold doing this:
Google Image Result for http://ashtangayoga.info/asana-vinyasa/primary-series/25a-Upavistha-Konasana-A.jpg
Those are (1) Don't bend your back, (2) Dorsiflex your feet, (3) When the pain becomes too much, push your legs down into the floor and straighten them as much as possible, and the pain goes away.
If you practise this you will get better at the traditional box splits. I only do yoga for about 20-30 minutes a time, five times a week and I am not too far from the box splits and can do the front splits if I warm up properly, despite the fact that I almost never specifically practice those exercises. I can almost do them because my fundamenetals (or foooondamentals, for Mucha Lucha fans) are good.