In training:
Whatever you do, do it gradually and progressively - your fitness will not improve instantly but it WILL improve and after a few weeks you will start to notice a real difference.
Start slow - warm up, build up - finish strong.
Finish each run feeling as if you could still do more.
Listen to your body - in time you will learn how - pay early attention to any aches or pains.
After each run rest, recover, replenish and rehydrate.
Keep at it - don't let any setbacks get you down.