IMHO, 5ks are the perfect distance to use as cross-training, maybe with a 10k thrown in every now and then. I mentioned earlier that I did a 10 miler and it was probably one of my more boring runs. After 30 minutes or so, I'm ready for it to be over. I still don't know how people train long distance day in and day out. 5k requires a decent mix of speed and aerobic fitness, you'll get the enjoyment of doing a few races and it shouldn't be too hard on the joints. It really shouldn't impact your strength training too much, either. Tracks are fun. Always used to love speeding up in the turns. The nice thing about Higdon and other 5 k training programs is you usually have an interval/speed day each week. If you want to increase it to 2 days and drop a longer/slower day, I doubt it would hinder your progress.