For those of us that unfortunately have left our teens behind its still something to think about.
For a while know ive been doing high intensity training.
First, you warm up for a couple of minutes with some gentle cycling: then you cycle as fast as you possibly can ("hell for leather", as Mosley describes it) for 20 seconds. Then, you cycle gently again for a couple of minutes while you catch your breath, then do another 20 seconds "flat out". Then, for a final time, two minutes gentle cycling to catch your breath, followed the third period of 20 seconds at "full throttle".
I did it just to see if it had any benefits after seeing it on horizon and remarkbly with that little exercise I've seen an improvement.
Since then ive done the couch to 5k.