Heh, get the extended warranty on those new legs...
I had that feeling a couple of years ago. Weak, just no power in the spring, even though I got some miles in during the winter. "Is this it? Am I just too old to ride like this?" Then I slowly got back to a normal pace and climbing ability, and the summer was fine.
It didn't happen this year, I've kept going okay. Good! Why that year and not this one?