Hi
Well you certainly aim high!

I think all things are possible, but something like a Marathon des Sables would be a major project and need a hell of a lot of work
To be honest, when I was running 70-80 km per week ("standard") marathon training, I really didnt feel that I could have handled that kind of several-day long distance run. You should, hopefully, get some advice from some of the people here who have done that kind of event. I suggest you try, eg, a normal marathon first, see how you go, then look in detail at the sort of training & stamina you would need (long-distance running, hill running, sand running, running in strong heat, running over several days, running with a backpack... :eek: ) Try to plan for the debilitating effect all these would have on you in combination; these runs are as much a mental challenge as physical.
It is likely to take a couple of years plus to get to a comfortable standard for extreme distance from half-marathon level.
I think that if you did do one of these, you would need to substantially increase your miles, and if possible, lose a few kilos, but I certainly would not rule these races out as impossible, since I don't think it is impossible, and it's a great goal to have
Best of luck!