I've been sufferring from UC for 10 years, but have run almost every day during that time. I found the biggest thing is to get my hemoglobin and iron (ferratin) levels checked regularily. This is because iron is needed to carry oxygen. Blood loss can happen during UC, leading to low hemoglobin and iron, then running is difficult because oxygen is not reaching the muscles.
For a guy hemoglobin is suppose to be from 14 to 18 I believe. I am ussually around 16. I have been up and down over the years but found running performance decreases once it gets near 14. Have to maintain around 16 for good running. Different for everybody though. Adult women should range from 12 to 15. If you are much below 12 it could affect running.
Even if hemoglobin is normal Ferritin levels can be low. I think ferritin disaears first. Because of UC my ferritin is on the low side (range is 18 to 270 for men) but if it goes down below the lower limit... then running suffers even if hemoglobin is normal.
I take iron supplements regularily to keep iron up. Have found during a flare that they can be hard to take, but Flinstones with iron seem to work.