Do You Plan? Or Just Go?

Today nor tomorrow are my run days, but Tuesday I have my "long" run planned.  I'm not sure how long it is actually going to be and that is what I'm debating currently.

I tend to go into runs with a plan.  I'll run this far for or try and get to this landmark.  I won't walk until whatever mile... etc...  You get my point, I have a plan.

It's actually kind of sad how often that plan dissolves within the first mile or two of my run.  Heck, there are even mornings that I don't even get out because I am just so tired.  There are days I'm still sore or my muscles are just tired and I end my runs earlier than I plan.  Then there are the really good days where I get out there and I just feel so good I run right past what I was planning to run.

That has been the case the last few weeks.  I got 5 miles in when I planned for 3.  And then I ran 7 miles last week when I only planned for 5.  However, I'm also well aware these feelings are fleeting at best.  I might get them a handful of times.  My last two runs haven't felt that great so I'm really not expecting too much from my run on Tuesday.

So I'm planning for a 5 mile run again, and we'll see how that actually goes.  Will I surpass that?  Give up early?  Or hit it exactly?

So what do you do?  Do you plan your runs before hand or do you just decide to get out there and see how far you can go?