I was out 12 weeks with one respiratory infection after another. I lost enough weight that I fit the medical standards of "underweight"and "anorectic!" I've really only been able to run this past week and have finally made it up to 3.5 (slow) miles. But that gives me a reason to explain how I build mileage.
The first day back, I do whatever I can. After that, every day I head out and, if I run the same distance as the day before in less time, I stop; if not, I run 10% further (to whatever seems like a good place to stop, usually the nearest 0.5 miles). This seems to work well for me for a few weeks, when I can start thinking about pacing and actual plans. It usually falls apart when there's a bad stretch of weather - and a blizzard is predicted for tomorrow.
This time, however, I've had to make another concession to getting older. I follow the plan above only every other day; alternate days, I run whatever feels comfortable. I have a 4 mile race scheduled in 19 days; I only plan to finish,not race.