Well. Nobody is answering so I'll take a wild stab at it.
<snip>... and am wondering if anyone has experienced weight gain with endurance training.
Yes. Back when I tried to say in near constant half-mary readiness (upwards of 150 mile months), I had all sorts of problems trying to keep my weight down because I always felt ravenous. At the time I thought I was about 10 pounds from my ideal weight, so I tried
to stay in a calorie deficit. For me, it was very difficult to maintain a heavy running schedule in a calorie deficit. Also, I didn't eat a very clean diet and ate way too much junk food in response to being so damn hungry all the time -- this didn't do me any favors.
After reading your post I just think you need to keep tweaking your total calories down a tad until you are back in a small deficit if your goal is to slowly drop just a couple of pounds while still maintaining your current running fitness. You say it seems like 1700-1800 should be an ok number, but if you continue to gain weight it could be a tad too high. You may now be a very efficient runner and your body may not be burning as many calories as you think it is.
Drat your super fit efficient body.
Good luck to you!