Say you like doing cardio and do 3 - 4 hard sessions a week with distances from 3 - 10 miles. What if you were to also weight train twice a week with a legs,chest,triceps and back,shoulders,biceps type approach i.e. sort of like a full body sessions.

Is it theoretically possible to gain muscle and get fitter simultaneously if you eat enough calories?

I would say it’s actually realistically possible … not just theoretically. It happens in athletics all the time, especially with sprinters.

But I would also say it’s also fairly hard to do on your own. Much easier to focus on one goal at a time.

You could get “fit” you could be in amazing shape this way but, and its a big BUT you will not be massive by any means and you will not come close to reaching to reaching your potential in strength. But yes in shape and fit is possible. If your fine with that then its great.

Yes, I should have added…

If your goals are anywhere physique related: Brad Pitt in Fight Club/Snatch/Troy; Daniel Craig as 007; Wolverine/Thor/Batman… anything like that, where it requires adding muscle and also being lean…

Then a more standard approach is [lean] bulking where you focus mainly on adding muscle with some fat, and then cutting where you remove any remaining traces of fat.

it is absolutely possible. It’s called being an athlete

easy, no. possible, yes…