At nearly 20 weeks, as a high-level amateur athlete, I am:
Cycling - up to 4 hours on my road bike. Most rides are more like 1.5-2.5 hours. Not racing anymore though as I'm just too slow. I am very confident in traffic and in general but will stop if it gets too uncomfortable or I start to feel the bump interferes with my balance and reflexes.
Running - up to an hour or 8 miles (only because I just started again when I got pg so been building up steadily again). Wanted to do some weight-bearing exercise plus in winter I quite enjoy it.
Swimming - will take this up again when cycling and running are no longer options.
Regarding intensity, I had to cut back in the first trimester just because my body didn't let me (not that I'd been pushing myself anyway). Energy was very low so some days were just about getting out and doing something, regardless of how easy or slow.
Second trimester I've had more energy but now I'm finding that the extra weight and encroachment on my lungs is slowing me down again. Again, just listening to my body and going with how I feel.
Here's a good article for anyone questioning whether a good amount of exercise is healthy in pregnancy. There's a lot of unfounded crap out there ("keep your HR below X beats" etc.) so worth reading if you're a fit active person before and want to keep it up during pregnancy. All bets are off if there are complications of course!
www.ideafit.com/fitness-library/the-pregnant-athlete