need to resynch fitbit and MFP, you can uncheck "negative calorie adjustment" under settings if it messes you up, for me i either ignore it (on my rest/strength training days) or it motivates me to move more to reduce the negative number

Another thought though, since you are doing marathon training, and your actual TDEE on some days is going to be massive compared to rest days. Which you will need as long runs are longer.

The weekly average TDEE method won't work that well actually, as you'll be badly underfed on those long run days, and eating a pretty decent surplus on rest or low burn days.

You do need to sync accounts, negative and positive.

Set MFP to maintain, no weight loss goal.
Set activity level to probably Lightly Active. Go higher if you find rest days still have big positive calorie adjustment. If slightly negative that's fine.

Now your eating goal is your TDEE after Fitbit syncs occur.
Take daily eating goal x 0.85 to see what you are really going to eat that day.
Or take it x 0.15 to see how much to leave in the green uneaten, is what I actually found easier. Write it in the diary notes.
Memorize your minimum protein and fat goals desired, who cares carbs, you'll need those extras.
No, you don't need 30% protein on days with say 4000 calorie burn. You need carbs so you have energy for the next day.

Still manually log workouts, could be in MFP now if desired, Or Fitbit, then just make wall post in MFP to inform FL about workout.
But use the more accurate calorie burn from HRM for the cardio for that manual logging.

This will result in you taking 15% deficit daily. After a week or two you'll know about what each of the days will have as daily goal, except long run days will get bigger and bigger.

And figuring out the 15% to leave green gets easy too. 10% is easy, then half that added on. Keeps the brain sharp with the mental math.

That way the big burn days get the extra carbs you need back, rest days still have a deficit.