Marathon or Decathlon Training Questions

I'm just curious and this is a completely hypothetical line of questioning. I'm curious how high your deficit calorie-wise is suppose to be for any given day? If you were say training for a marathon or decathlon where you'd in theory be losing thousands and thousands of calories everyday. For example I put in Running and one I chose as an example is Running (jogging), 5 mph (12 min mile) this for 60 Minutes for me shows I'd almost lose a 1000 calories. If someone did this and compiled biking, swimming, weight training, etc. on top of it I'd think it be almost impossible to healthily "eat back your workout calories."

I've been to the point a few days now where I've gone over the 1000 calorie mark and certain future aspirations of mine have made me wonder how to do such things? I've learned many lessons already with not eating enough calories and have now stabilized what I'm doing and not allowing myself to go crazily under my suggested daily caloric intake.

So mainly my question is how do people training where they're losing say 2000+ calories working out a day wind up eating it back healthily or is this more or less just what pro-athletes do with help of physical trainers, dietitians, nutritionists, etc?

Thanks in advance for any help/feedback.