My advice is to ease back into the training slowly. Three weeks off means three weeks return. So you shouldn't do any hard paced workouts for at least the next three weeks. And the mileage during those three weeks shouldn't be jumping into whatever plan you were using where you would have been 3 weeks ahead (like if you stopped running at Week 12 of your plan, you shouldn't plan on restarting at Week 15 this week). Instead do something like 60% of your recent peak (like if 20 miles in a week was recent peak, then do no more than 12 total miles) in the first week and then maybe 70-75% peak for the two weeks after that. Again, all at paces far far far slower than your estimated Marathon Tempo pace.
When you return to running this week, you're going to feel like you lost all your fitness. You didn't. According to Jack Daniels calculation, you can estimate you lost about 5% of your fitness level during these three weeks (with a 20% loss being a complete detraining). It just takes time for your body to reacclimatize to the training. So be patient and don't force it. That will pay off in the long term.