And now for a brutally honest confession: I spend a tremendous amount of time and mental energy finding excuses to skip my workouts.

These excuses don't have to be plausible. Lame, sketchy, ridiculous reasons work just fine. The wind gusts are 15 mph? Probably not safe to ride my bike. (Also good are too hot, too cold, 10% chance of rain, etc.)

I'm on the waiting list for spin class? Smarter to stay home than set myself up for disappointment. The gym will be packed, I'll never get a machine, it gets smelly when it's too crowded. I have a cold, deadline, important phone call, headache, dinner to make, shopping to do, drinking to do, TV to watch, funny sensation in my little toe.

You get the picture. I'll sweat, but I do so grudgingly and when conditions are perfect. With the exception of yoga, I have to force myself to exercise because I don't have much fun when I do. For years, I've blamed this on my essential personality. I'm a wuss. I lack that x-treme sport gene. I prefer surfing the web to venturing into real surf. I forever seek the easy out, instead of ever seeking tough physical challenges.

And then, last week, I tweaked my knee at my favorite new hangout, the rock climbing gym. Whatever muscle or ligament I pulled really kind of hurt, but I decided to ignore it. Two days later, I went to the gym again, strained my knee again, ignored it again, and so on until I ended up at the drugstore, buying a giant bottle of ibuprofen and an elastic knee brace.

Still, I kept climbing, covering the brace with my favorite Prana pants. See, for the first time ever, I want to minimize -- rather than maximize -- the discomfort caused by a sport. I'm trying to suck it up so I can keep climbing.

This is a complete and total revelation to me.

My injured knee is, of course, crushingly and horribly painful when it comes time to ride my bike or go to the gym. But it's nearly forgotten when I want to climb. Ah, the wonder of selective pain tolerance.

Knowledgable friends of mine -- doctors, massage therapists, yogis, cyclists, joggers, anyone with a modicum of sense -- have strongly urged me to do nothing more ambitious with my knee than RICE (Rest, Ice, Compression, Elevation.)

But yesterday, I got my first opportunity to climb outside, and I couldn't resist. So I went up (well, half way up) a real rock wall for the first time ever. It was fun, scary, hard, painful, and fun. Of course, I'm paying the price, and I do need to let my knee heal fully. As I write this, I've got a bag of frozen carrots on my elevated knee, an ice cuff on my left forearm, a bunch of scrapes on my hands, and a huge smile on my face.

I finally understand what exercise junkies have known forever and what self-proclaimed couch potatoes may not yet realize: Exercise is addictive -- but only when you actually enjoy it.