1.5 mile swim

When I decided to do a ironman tri my original fear was that I wouldn't be able to run the marathon. But as I started training the running was progressing well. However my swim was not up to speed, so the fear switched to the swim. After swimming twice a week for a few weeks now I am up to doing a 1.5 mile swim and it seems adding in an extra mile would be no problem. But now I am back to being worried about the run. My knee has been sore and swollen since my first tri even though I haven't hardly been running much lately. My last two mile run caused a lot of pain. Somehow I need to get past the knee pain problem.