A great thing with FK too is that you can ‘break’ the arm joints, ie have them bend backwards for a frame or two to show extra flex, it’s not realistic at all but you see it a lot in classic cartoony walks. You can kind of approximate it with IK arms if you key the pole vectors just right but it can cause some weird looking deformations with the wrists depending on how the rig is set up.
IK legs come with their own challenges too - with a walk the poses that give you trouble are the contact (front leg) and push-off (rear leg). To get a good sense of weight the legs should be as straight as possible on those poses but with IK when you transition from a bent leg to a straight leg you often get a knee pop. Some people have come up with systems (often called 'soft IK' or 'snap damping' (not to be confused with soft IK in C4D which is something different!)) that try to lessen that pop but personally I find those just as bad as they take control and often prevent you from getting exactly the pose you want on the frame you want.
So what I do is use my ghosting tool or a screen marker (current favorite is Epic Pen, free for windows) to mark the position of the knee on the straight pose and then use scaling on the leg to fix the spacing. Fix the spacing and you fix the pop :) ~I think I cover that in my walkcycle tute video.
Hope that makes sense and is some use!
One other thing to keep in mind - many people think a run/walk is a good 'beginner' exercise for an animator, but it's really not! It's actually really hard to pull off a convincing one. Bouncing balls are the place to start really if you want to build solid foundations, everything you learn doing those will apply to a walk and all kinds of other actions.
I do walks and runs all the time and have never created one I'm 100% happy with.. I keep trying though, here are a couple I did a while back.
Generic run. FK arms.
'Attitude' walk, based on the example in the Eric Goldberg book. I used IK for the arms in this as I was more concerned with the poses than convincing weight, you can see there are a couple of frames where the elbows lead the wrists though to try to get that sense of 'breaking' the joints in there.
With this walk there’s no attempt to make it reaslistic, it’s more about trying to show strong poses as long as possible and whipping through the breakdowns as fast as possible. Each key pose is ‘favored’ very heavily.
Oh and one more I guess
I got a note to let his arms swing forward more which I didn’t address yet, but other than that I’m about 50% happy with this one (which is good for me!). I’m using FK arms in this one and I’m breaking the elbows too for a frame as they swing forward.