I'll chime in here (I am a personal trainer)...to increase your cardiovascular fitness you must increase and decrease your heartrate several times over a period of time. MTBing does this naturally, but you can simulate it at home...but be careful of not pushing your heart rate high enough, for long enough...or "adapting" (true technical term).
For example...I am a professional rider (of horses). I can ride along for some time, thinking of other things, and barely break a sweat. A beginning rider is using muscles that they have never used, so posting a trot for 10 minutes leaves them breathless, with their heart rate somewhere near Mars. Take that same rider who is totally worthless (fitness wise) and stick em on a MTB and, if they are proficient at MTBing, they can go longer and faster easier than someone else who can run a marathon, but has never been on a MTB.
So, if your exercise is MTBing, you have to continually fight adaption to truly become fit. Pedal harder, faster, stand more (if you tend to sit) or make yourself do seated climbs if you prefer to stand. And, to get fitter, allow yourself to recover, then hit it again.

And here is my personal hint...I can't stand to hear myself breathe. It makes me feel way tireder (is that a word??). So I iPod it. I use one bud unless I am on a deserted trail or road and don't listen to myself pant. I use a HR monitor only while spinning. And before you know it! Fit!