It happens to everyone, Sam.
I was happily fooling myself I was still fit, and making excuses for why I hadn't done any events since September 2007 (very busy, don't you know...). I stopped commuting every day because I just didn't feel like it, and before I knew it there were whole weeks where I wasn't on the bike. Then a whole three months when I was in Brazil this year. Sure, I ran a bit, but it isn't the same. By the time I stopped to weigh myself, I was not happy about it. and it was pretty obvious that I'd put on a bit of weight (not to most people's eyes but they don't see me with my kit off!).
Anyway, what was the answer?
New bike!
And maybe being back in Tokyo helps too. Riding my evil black singlespeed around the streets and pushing against the wind off the bay, along the riverbank paths, in the sticky, sticky heat (even at 6 in the morning) is great. I've been making sure I am out at least 5 days out of every 7, and do at least 40km each morning, two or three times that on the 5th day. Hard and fast too - these are weight loss and conditioning kilometres.
Now I'm heading down the path to 40, I'll probably never have the climber's body I had for those brief few years in my early-to-mid thirties again (some friends said I was looking too thin... and perhaps I was), but there's no reason I can't still be properly fit and capable of tackling the big hills again.
And come September, when we settle down in Canada, there will be new challenges...