Having My Cake and Eating it too.

So after a long, long time and 40+ pounds, I have finally decided that SOMETHING has to be done about my aversion to exercise. A hundred months ago I decide to start running again (something I haven't done since 1998), and when you're running, it feels like flying sort of, so I thought it would be fun. That escapade ended with me begging the chubby dog to pull me back up the hill to our house...so I know running is out of the question this time--at least initially.
I decided last night to start walking. I took the chubby dog with me, and he and I did a very brisk walk. Everything from my house is uphill, and if you're not careful where you go, you have to come uphill at the end too. So I decided we'd do a loop up the street a bit, onto another street and then connect with the street that eventually reconnects at the top of my house and we'd come back down (that's the only downhill way I know!). So chubby dog perked his ears and we took off. Five houses or so later, my thighs were killing me and the i-Pod did nothing to distract me from the reminders that I used to walk everywhere in town (unless my friends were going the same way/place) because I was a deprived child without a car all through high school (another story, another time!). How is it that 15 years later I can't make it up a hill past five houses?
I did do the whole loop (in about 1/2 hour--I say it was about a mile, Hubby couldn't even give me that--where's the loving support?) and didn't collapse coming in the door to my house. I didn't even take a break--once chubby dog gets going, there's no stopping him!
I grabbed a bottled water, and realized that it has expired. How does water expire? I always thought that water tasted boring to begin with--if it's expired does it not rehydrate the body? Will it actually work in reverse? This panic lead me to eat a large piece (okay, 2 pieces) of cake. Now I probably need to walk the stinking loop twice and run it once. Now I'm panicking about how on earth I'm going to ward off the calories and how I'm going to make the pain stop and how on earth am I supposed to motivate myself to get up and get moving?
Maybe I need another piece of cake to debate this.