Winter Weight Gain : A Paradigm Shift?

Stepping on the scale, I recently have been wracking my brain on the reasons for my recent weight gain.  I have fluctuated slightly over the last year, because I naturally plateaued, which is fine.  But as of late, I noticed the number slipping back up.  I noted it to my friend and my wife.  In my head though I began to recount the new snacking I have been enjoying in the evenings.  This must be the culperate.  Or the few chocolates I enjoyed over Christmas.  I beleive this is part of it.  But there is another part that I needed to face.  Its the part of my new regiment that I couldn't factor in with out another's opinion and observation. 

What my beautiful wife pointed out was that my 3-4 visits per week to the school gym for my new gym program on top of my regular runs with students may be the culporate.  DUH.   However, my thoughts are, how much is from muscle tissue?  I am in a unique situation for myself, because I have not wanted to be huge, just fit.  Will this affect my times in a negative way now?  IS it really from the workouts in the gym?  I think trusting the process has something to do with this.  I am a pretty clean snacker.  Whole grains, fruit, yogurt, milk.  These I have replaced from the old wings, chips and nachos from days of old.  Being in an unhealthy paranoia is not one of my 2010 fitness goals.

Keeping healthy and active is my life time goal.  Not paranoid.  I will continue to allow my food to be enjoyable, sensible and reflective of my activity level.  Once training for a timed event starts, so does changes in my body's needs.

This is where people might begin to roll their eyes and think, "nice problem to have".  But what I ham facing with this paradigm shift is the fact that our fitness and dietary goals and needs constantly shift and need to be re-evaluated.  I am going to need to dig deep, face the previous paradigm and rely on the facts of the new activity level I have.  I will be researching this in other's running info.  Has this happened to anyone else?

Supplimentary:
Here is an interesting article on gaining weight while training for a marathon.  Fodder for thought.  A running friend of mine feels that I need to attribute how much you need in terms of caloric intake to what you are actually doing.  It could be both increase in muscle and fat both.  Any increase effects running times, as you have more to move if you gain.