Good technique and running with a relaxed body is key to running easy for a long distance.
No matter how tired you are, it should always be possible to run with good technique.
A large part of bad technique is mental tiredness and lack of focus in the last part of a long run. One way to train to run with tired legs without actually having tired legs, is by adding some weight to your ankles.
Put 2-500 grams on each ankle to make your feet feel like you have been running for a much longer time. But be aware that you are now using your muscles in a new way, and that you should progress slowly.
Preferably start with 15-20 minutes of running with weight on selected training sessions. After you have run the given time with the weight on the ankles, you can put the weights in a backpack for the rest of the run. That way you will also get more used to running with something on your back. Then slowly extend the time with weight, but keeping it to a single run every week.