My other half has to do one as part of the fitness test for her job. She can get to level 6, but not a lot further.
I tried it with her while she was training. I was still going at 11-something and stopped as I still had a full gym session to do, the other half was getting annoyed that I wasn't keeling over (and secretly I was close to keeling over).
Always been a bit of a runner – just a shame my running base stamina that seems to have remained quite well from my school/uni days doesn't seem to help me ride my bike quickly over any major distance!
EDIT – As for improving your result, you need to improve your base cardio fitness, but also do at least one speed/interval session a week. Even something simple like 5 mins to warm up and then sprint between lamp posts, jog between to recover….3 sets of 5 or 10 depending what you can manage and then 5 mins running to warm down.
Or an interval session where you mimic the bleep test by ramping up the pace every x seconds with a resting period in between sets.