Kris, like you I started with a 70-300 and moved up to the 400L. I am in Colorado and there are a huge variety of opportunities for wildlife photography here. I started off not knowing where to start and wound up just driving from park to park to see what I could find. I got a few pictures, but they were very hit and miss.
I then started looking more closely at some of the larger parks with more secluded areas and started seeing more and more wildlife. All of these were hiking expeditions. It involved either getting somewhere the day before and camping or getting there before dawn to start the hike. Personally, I like hiking around, mostly because I am not a very patient person and would have a hard time sitting in a blind waiting on subjects to come by.
The one time I tried to stalk, I wound up getting an added bonus. I was driving one day and saw 2 hawks on the ground. Not a common occurrence so I stopped and grabbed my camera. I usually have a 150-600 on it for just this, but this day, I only had a 70-200. A great lens, but I would not be able to stand on the side of the road and take pictures. Luckily, there was a large tree between me and the hawks, I used that as cover to walk up on them. Just as I was getting in to position to take pictures, they both took off. I ran around the tree to try and get some shots and as I was looking for them, a bald eagle swooped in! Bonus! The main point I am trying to make is that you need to be open to all opportunities for wildlife.