I have to qualify my statement by saying that I do love to run. The thing that drives me nuts about it is that some days 5 miles is really easy. Other days, 2 miles is work. On the days that 2 miles is work, if you cut the route short, you feel like garbage mentally. If you gut it out, then you physically feel like garbage. Either way, I feel like I can't win.  I think the important things with running are 1) make sure you have the right shoes; 2) have someone do a gait analysis on you to insure that your form is good (do you pronate or supinate? do you need arch support? do you run on the balls of your feet? etc). 3) vary the work out - hills one day, interval training another, distance on a third day.  All told, I enjoy bicycling more than running, but sometimes I don't have 2 or 3 hours to get a nice long ride in. If you've only got 30-60 minutes, a run is a much more efficient work out.