Wow, I feel like I've never felt this good in my life (though I know I've felt better). Just got back from a 30-minute run. Here's a few tips that I've learned over the few times I've tried to get back into shape after spending a period of sporadic physical activity.
1) Go for time, not speed. Tell yourself that you're going out to get 30 minutes of activity, not that you're going to try and run 7-minute miles for as long as possible. Especially if you've been in peak shape before (I have), you're going to feel frustrated when you get tired quickly. You're going to get a better workout if you stay at it longer; you'll build more cardiovascular endurance, and burn more energy if that's what you're looking for.
A few days ago, I went on a 35-minute run and took my heart rate when I stumbled back; the fact that it was over my training zone was a sort of wake up call to me. The body just can't go from nothing to speed demon.
2) Take it easy on hills. No reason to give yourself an extra reason to validate cutting a workout short by killing yourself tearing up a hill.
3) Vary your workouts. There's no better way to generate boredom and disinterest than by going out and running the same route over and over again. Never run the same route two days in a row. Try and throw in a little biking, aerobics, swimming, or club sports, or skiing or snowshoeing if it's the winter and there's snow.
4) Don't stop. Don't be afraid to slow down if you're feeling like crap, but if you start walking in the middle of a run, you're not going to get started again.
5) Don't make excuses. While you don't have to go out every single day, tell yourself that you do. It's really easy to say "I'll do it tomorrow."
6) If all else fails, run with a friend. Even those with the discipline to force themselves know the benefits of working out with someone else. When you work with a partner, you naturally push each other.
7) Don't give up. It's frustrating to not be able to do what you could once do, but it will get better. Heck, I'm still in the process of working my way out of my sporadic attempts to start running again this winter. They all failed because I was going for speed, and not time.
Being in great shape is a great feeling, and something that I'm looking forward to being able to experience again. Hopefully things will continue to go well.