Long Stay Visit to France Without Visa

There are no systematic border controls within the Schengen area. There are occasional spot checks, but the odds are low. Note that the UK is not in Schengen; if you want to travel there, your passport will be checked in both directions. If you travel by plane, an airline employee will usually check your passport; if your visa is not valid, you may be denied boarding, with no recourse (I've seen it happen).

While you're in France, your papers may be checked by any police officer for pretty much any reason (basically, whenever something potentially illegal is happening, whether you are suspected of any wrongdoing or not). Various administrations and others may ask for your passport and refuse to provide service if they notice an expired visa (most non-government institutions wouldn't care, though). You could be deported if your visa is expired. You may be fined (even if you're leaving, I think). You wouldn't be detained if you were already leaving (but you may be if you're caught in the street). However, if you want to come back later, having overstayed a visa is usually a black mark on a visa application.

If you go between countries of Schengen Agreement you have no border to cross, so likely there will be noone there to stop or check you. Ports might be a possible exceptions. See @mindcorrosive comment.

When you leave France if your visa has expired you are risking not being allowed back into any countries of Schengen agreement for violation of the visa conditions and likely to be fined, here are some of the first hand experiences on the subject, and another one and more official like consequences.

My advice: Since you are going to stay and work for 7 months in France have you considered asking the company/organization that hired you for assistance in obtaining a long stay visa for both you and your boyfriend?