i'm not sure what cardio you're accustomed to doing, but many boxers supposedly run 1 mile and then sprint for 3 minutes to replicate the burst of energy they would need for a round. they repeat as many times as they can, or whatever their goal is for that particular day. of course this will increase your speed and endurance over time also.
wear thin layers and get you some under armor or the equivalent and shoes that fit properly and are not 6 years old. i don't know what it is but some guys can't stand to get rid of their old shoes, like they're woobies or something.
of course you could also run/walk interval if yr not accustomed to running.
If you live in the NE part of the country don't try this until it goes above -35 or whatever it is now.