Hi - DDCC here - just want to say that I was an avid runner pre-pregnancy; I'd been running 7-9 miles most days, and as others have mentioned, I was totally hooked on it! While going to bed I'd fantasize about waking up the next day and getting out there.
I was very sedentary my first tri, just cause I was so exhausted and got very depressed. But I picked it up again at about 12 weeks, and I continued to run (much less mileage, much more slowly) up until the 27-week point, when RLP just got to be too painful. And the whole time I WAS running, it was very tough for me, especially given the near-constant urge to pee, the RLP, and the tendency to feel fatigued sooner. At this point (I'm 28 weeks), I'm sticking to walking and yoga. Which KILLS me cause I love running more than anything.
I've heard a maternity belt can help with RLP and bladder issues, and I might try one out, cause I love to run so much!
This has just been my experience. I think if I had pushed past the fatigue and depression of my first trimester and kept running, I might have had an easier time throughout, and would still be running now. So if you can keep it up, I say go for it! My midwife and nurse encouraged it with lots of enthusiasm. They did caution, however, that pregnant runners are more prone to injuries of their pelvic symphysis, so DO NOT "run through" ANY amount of pain you experience. An injury to this area can take years to heal!
Aah, can't wait for the PP period so I can start running again. Good luck ladies!