I was never a fan of cardio machines untill my knee started playing up due to too much running.

Now a massive fan of intervall training on the treadmill, I recommend it to anyone trying to improve cardio fitness, the jog and sprint (or even walk and jog)

Also the concept 2 rower is an awesome piece of kit once set up properly, Again intervall training.

Personally I live near a great trail run, down by a quite river and over open fields, I have various runs ranging from 3 to 16 miles and also great to mix it up with mountain biking.