Is it supposed to feel like this?

I have never trained for a marathon before, so I don't know if what I'm feeling is normal.  But I do know this:  I feel tired.  Not just like I feel like I don't get enough sleep.  I'm physically tired.  Epecially when I run.  I just finished the longest 4 mile run of my life.  I seriously felt like it was never going to end.  I just shouldn't be that hard.  Four miles should be easy peasy compared to 15 miles, right?  And last Saturday when I did my 12, that first 5 miles felt never ending and my I needed to actually push myself to just get my legs moving.  I can see why that might be necessary after running a while, but it shouldn't be that hard to go out and run the first few miles.  My whole body feels like it's working so hard to keep going and I don't love feeling like I'm going to fall over and die.


I went to the doctor a few weeks ago to get some blood work done in the hopes that I'd find out that I had low iron level or vitamin D or hemoglobin or something that would explain why I'm feeling so tired.  Everything turned out just fine though, so I can't blame my physical condition on sub-par blood work.  I think the doctor thought I was crazy when I was complaining about feeling tired while running 10 miles.  (Duh, you're running 10 miles, of course you're tired!)  The doctor did give me a prescription for an inhaler because she said maybe I have exercise induced bronchospasms based on me saying that sometimes I just feel like I can't take a deep breath after running for a long time. Still haven't picked up that prescription... I just don't think that's the issue at all.

So what is it?  Am I hitting the proverbial wall?  Am I just being a wuss?  How do I fix it?  What if I don't fix it in time for the marathon?  Questions, questions...