Punching bag? Or start going for long walks. Make sure to keep the pace high enough that you have to give slight focus to the walking, that takes away some (not all) focus from inner pain.
Running is alright, but if you are not in good shape, you can't do that for long enough to subdue the need to cut.
maybe not cutting? honestly theres no point by the end of it you'll still be depressed but now you will also have a sore arm. instead work on how you can make things different and put everything into making it so.
You can also use other ways to relieve the pain like holding ice cubes, lifting weights, screaming, running till you can't anymore, destroying something...
My sister used to draw butterflies in her legs and upper arms each time she felt like cutting. She used a permanent marker and if the butterfly completely faded before the next time she wanted to cut she said it lived and she would write in little journals about them so she could look back and feel better. I'm not a professional so I hope you seek help from one if you haven't already. I wish you love and light in a more positive journey.