I have been walking for 45 minutes at a brisk pace every morning with good results. However, it seems like I’m not getting the results as quick as I was in the beginning. I have been following this regimen for six weeks; do I need to up the intensity f...

That is exactly what you need to do! You are already walking for a pretty good amount of time, so you should increase the intensity in other ways. I would suggest you start doing intervals of running and walking. For example you can run for one minute and walk for two minutes. This will effectively raise the level of intensity at which you are exercising. You can start with just 20 minutes of this type of exercise and work your way up from there. This should be enough of a jump to get you losing fat again. Just the change in the type of cardio will most likely be enough to get you through you fat loss plateau.