if you're feeling tired, as in generally fatigued, rather than specific tired muscles, then it could be you're not resting enough or not eating enough.
you can get through a 20 mile run with no fuelling, but you'll pay for it in longer recovery time, and more fatigue. you can get through your weekly runs with poor quality sleep, but likewise you'll pay for it eventually. if you're running higher weekly mileage than you normally would, aim to get a regular solid night's sleep throughout the week. The odd broken sleep, or late night won't kill you, but regularly skimping on sleep is not compatible with marathon training.
i've heard that many newer runners run their long runs too fast. aim for about 10-20% slower than your marathon goal pace. for a 3hour marathon pace of 6:52, your long runs should be somewhere between 7:30 and 8:15 ish. ballpark.
long runs serve multiple purposes, including conditioning your legs to take a pounding for the hours you'll be out on the road, so time on feet is important. but they also help condition your body to utilise fat more effectively as a fuel source. this only works if you don't run them so fast that your body is forced to use stored glycogen instead. so, slow down a little on the long runs. and eat some quality carbs and a bit of protein immediately after your hard runs. nutrition is not that complicated, despite the thousands of seemingly contradictory opinions on it. each a load of carbs to replenish your glycogen immediately after your long runs, and otherwise eat a healthy balanced diet. 3-5 meals a day, small and often is generally good, but whatever works for you.
hope you're enjoying the running. and good luck with your first marathon. 