I've been running for nearly two years now and yep, you can gain weight this way! I actually gained 6 pounds while training for my marathon (no, it was not "muscle" it was flab). Gotta count the calories, especially if you are using supplements on your runs like the gatorade-type drinks and energy gels.
As for motivation and staving off boredom, I think the absolute best thing I ever did was take my dog with me. I have a 2 yo (ish) jack russell/pit (?) mix (65 pounds of sheer muscle) and he LOVES to run...I get the added bonus of a quiet house for the rest of the day. Also, during the run I am so focused on keeping him in line and looking out for potential distractions (e.g. bunnies) that the miles fly by without me noticing. AND my time has improved by almost a minute in just two months of dog running
If your going to run with your pup get a harness (safer for both of you) and put some sort of protection on their foot pads, especially if you're running on pavement (I use "Musher's Secret", a wax-based goo that gets rubbed in).