Long distance practice

The day of Rotorua Marathon is coming. The left time to practice for this big event is only a month. Since I don't want to fail this race, I ran long distance today. It might be approximately 10km from Britomart to St Heilers Bay, so I ran total 20km. It was harder to keep running than I expected. I sometimes stopped and took a rest for a second. My objective is not to walk until I finish 42.195km. My leg is, however, not ready to keep running for 42.195km yet. I need to do long distance running once a week to achieve my objective. I always feel anxious about the lack of running, so I tend to overrun. Actually I also need to take a rest effectively not to get more and more fatigue. It's difficult to manage. Originally I had to make a running plan, but I didn't affort to do it. I just ran and ran without thorough thinking. I have to do it when I take part in next race.