To improve your bleep test score, try something called 'fartlec' training.
It consists of running as close to flat out as you can for say 20 seconds, then jogging at a very slow pace for 50seconds, then fast again for 20, then slow for 50.
The numbers above are just for example, adjust them for your personal requirements! You can use other markers such as lamposts, or steps, anything that you like.
Its a very good way of improving fitness, I usually do this twice a week. Compliment this with a normal paced, but slightly longer run once a week and you should see a good improvement fairly quickly.
Also do some swimming, very good for you both in terms of fitness, and taking stress off of your joints.