Keep in mind parliamentary elections also have the two round system - and in past elections a lot depended on strategic decisions made by the parties to withdraw their candidates in their second round in favour of someone else (generally that means against the FN), or not.

Still, it would be extraordinary (and not in a good way) for a 'party' like En Marche (note the initials) to take anything like a majority of seats with its hurriedly gathered batch of neophyte candidates.

And it could really take an amazingly long time before we get to the point where she'd win more than 50 percent, honestly. The FN has gained a lot of support already, but in terms of realizing its voter potential it's getting close to its ceiling. I think it's more likely that the next president would be a LR one but with some obvious FN inspiration - Sarkozy seemed poised to play that role and might've successfully done it, if not for his sky-high negative rates.