Sup /fit/

If I eat, say, 500-900 calories per day and get at least 60g of protein, and I exercise all of the calories off (1000 cals/hr running), presumably I'd be at a 1600 calorie deficit with minimal muscle loss, right? What would be the downside to this diet? (I know I'd be able to stick to it, I was anorexic for a few years as a teen and I ate ~200 per day without bingeing).

Thanks.