It depends a lot on how severe your pneumonia is? I've been unfortunate enough to have it twice in the last 15 months. First time I had it was November 2011, and had it severe in the right lung and slightly in the left, and had to stay in hospital overnight. I went back to work in the January and set my sights on the Folkestone 10 mile in early April. The first few weeks of training were very difficult. I started off with a lot of slow short runs about 3 miles at 9 min mile pace and even these were tough. But I slowly built up the distance and pace and run a 1.15.30 at the race only 2 mins outside my personal best but I didn't have a very comfortable race with lots of pain in my back and chest, but got through it. I then ran a half marathon in may and I felt great all the way round and finished in 1 hr 35. Then in June I got pneumonia in the left lung and was off for another month and a half, and again just took it very easy.
All I can recommend is don't run until you get the all clear from your gp and have a chat with him/her about getting back into running. And when you do start up again keep it short and slow and build yourself up. It was tough for me mentally as it felt like starting all over again but if your persistent and positive it won't take you long to get back to full fitness.
I hope you get better soon
