Are you sure you are (1) fueling yourself enough while you are on the bike and (2) not over fueling yourself once the ride is over?
In order to ride well and not go into a starvation mode, you need to be sure you are getting at least 250 - 300 calories per hour ON THE BIKE. While you are riding. When you are done, you need about 400 - 500 calories of a good quality recovery food.
Beyond that, really watch what you are eating and be sure that you are not approaching your weekends as a free for all "because I'm doing these long rides". Be honest with what you are eating while you are off the bike.
I used to have a very similar story to yours. Weekends/long rides would cause a bit of a weight gain. Then something finally clicked. I started eating MORE on the bike and LESS the evening after the ride, etc. I have lost 11 pounds of FAT (per body comp tests performed by the same guy) in 2 months. I have stopped (for the most part - no one's perfect) saying to myself "Oh it's OK. I can eat this entire pizza because I had a long ride this weekend."



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but I've been falling into that "I burned thousands of calories today so I can eat whatever I want this evening" trap. 
That does put it really well. The idea of breaking it into the fueling and nutrition windows really simplifies things. sadly, my nutrition window really needs to lose some wine and beer. 

