start slow, not in intensity, but in joint protection. Swim, bike, rowing
Let your joints survive while you burn fat and put your body in a better position to lift or do hiit training.
Drink crazy amounts of water and cut out all refined sugar. No soda. I drink the wal-mart energy packets. It gets me caffeine so I don't have headaches. It's cheap. It's a pretty good preworkout as well.