what seems realistic at the moment of planning, becomes unrealistic when the unexpected things/events start putting sticks into your wheels .
Yes.
Plan, yes. But don't over-plan.
An over-planned time schedule spreadsheet is a good prop, right up to the point where it has to be tossed.
At that point, you loose the prop and are at a bit of a loss.
Have some more vague plans instead.
Plan to get to Brest before any sleep stops.
But that might not work out.
Backup plan, sleep earlier.
Plan to ride back as 2 x 300k,
Be flexible about where you will stop to sleep.
Don't be a slave to a schedule you made on a PC in the comfort of home.