I am going to play it safe and make an adjustment to my short- and long-term goals.
Physically I actually feel really good other than that one day I mentioned in the above post. However, I have realized that mentally/emotionally I have definitely been exhibiting the signs of overtraining. And I don't want to push it and cause physical breakdowns (injuries). Also, I am not sure I like this training plan I took up where I am running just three days a week most weeks, even if my total miles per week are decent.
So, after realizing that my long-term goals (looking a few years into the future) are to run well enough to easily qualify for the Boston Marathon and discovering that focusing on 10K training just may be the best way to get there, that is what I am going to now focus on. To be safe, I will remain at my base mileage (around 20-25 miles a week) for a month or two before I start focusing on 10K training. As mentioned earlier in this thread, I have been injured too much the last several years, so I want to do my best to ensure the chances of that happening in the future are small.
Also, I like the idea of getting faster and focusing on running well in 10K and similar races ranging from 5K to half marathon. Then I will start focusing on running a marathon. For me personally, I don't want to just run a marathon, I want to run it quickly. Of course, as with everything in life, I will remain flexible with my running goals and may change them in the future, but right now that it is what I have in my sights.
So, I will go ahead and end this thread here. If anybody has any comments or questions, I will come back here and post and answer them, but if this is the last post, I will simply let this thread slowly head back to the archives. Thanks everybody for reading and for the support. Even though I am putting off my marathon for a while, perhaps as long as years, do know that I am very vested and excited about my running - it just will be more focused on speed and shorter distances for the foreseeable future.