To work on strength training for the upper body and not the lower body seems like a poor strategy to put yourself in position to run a stronger 8k. If anything, I would focus on weight-free strength training exercises for the muscle groups that experience the most fatigue in race situations ie. abs, quads, calves etc. I think supplemental training in those areas can be a great aid to injury prevention and improved endurance. Take a look at the book from the link above, I found it to be a great guide.