When is it okay to give up?

Are we allowed to change our minds about our path in life?  Are we allowed to want to do something different?  Is it okay to realize that you like doing something that you're good at even though it's not as interesting and that you don't like doing the something that's interesting because you're not as good at it?  Are we really afraid of disappointing other people or are we more afraid of disappointing ourselves? 

I'm not doing the 100-miler this fall.  Decision has been made and I am actually happy with that.   With the slower-than-expected recovery, I'm not running more than 3 days in a row; while I think I could pull it off, it would also require everything going exactly as planned.  If I've learned nothing else over the last 10 years of my life, I've at least learned that I have about a 0.01% chance of everything going exactly as planned.  (Although, I think the wedding cake this weekend pulled that 0.01% chance off, more on that later)