Q: I used to like running. I used to run a lot. Then I stopped. How do I get back into it?
A: First off, you need ask yourself some questions. How long has it been since you ran on a regular basis? A week? Months? Years? How far and how often were you running when you ran regularly? A lot of how fast you can return to running depends on what you've been doing during your layoff.
The answers to these questions will help you figure out how to start again. If it's been 6+ months since you ran regularly for any kind of distance (I'm thinking more than 15K/week here) then I'd suggest pulling out the old Couch to 5K program.
Couch to 5K works wonders. It features reasonable mileage increases and is challenging enough for those coming back to running to enjoy it. You might be able to jump in at say, week 4 and then go from there. But the key is to take it slow. I'd recommend repeating the challenging workouts or even whole weeks, because it's better to play it safe than to overdo it. And if you've been a runner in the past, odds are you'll start out like a bat outta hell at first because of your enthusiasm and the chemical rush of running again.
I'd also recommend signing up for a 5K race 8-10 weeks out. It's great motivation to stick with a training program and enough to push you out the door on the days when the weather isn't perfect.
If it's been less than 6 months, then you can ease into a run/walk routine at a slow pace. The key is to get used to the stress of running without overdoing it. When I came back to running, I ran a 7 min/km pace (11:30ish/mi) and gradually increased my run walk intervals from 5:1 to 7:1 to 8:1 to 10:1 to 15:1 and so on. If I felt "off" or tired, I walked or repeated a run/walk interval before moving onto the next one. I never went over 32 minutes. The goal is to gradually increase your endurance while building a routine again. Don't worry about speed, it'll come.
No matter where you're starting from, there's nothing to be ashamed of when you're starting a training plan. Fellow runners on the streets, paths and trails won't judge you. Everyone needs to start somewhere, and you should be proud of the fact that you've taken that first step.