Here are some exercises that you can do to help make running easier. Doing these will strengthen the muscles specific to running, and help make each run a little easier.
There was a study done in 2008, and they found that doing squats 3 times per week improved running economy and time to exhaustion without any change in body weight. So, essentially, you can run longer without getting tired if you do squats 3 times per week.
This is where you lay on your back, with your feet on the ground and lift your butt off the ground. Start with 30 seconds at a time, and do 3 sets. As this gets easier, start lifting body parts off the ground, like a leg or arms. You can also increase your time.