enough exercise??

Stemming from the post "how much exercise" I am now worried I don't do enough exercise. Basically I have lost 5stone (70lbs) over last 2.5 years. The majority came off last year. So I started runnng to keep the weight off. In July injured my achilles and am only just getting back into it. I am trying currently to get out 4times a week (and succeeding) but only for about 35-50mins each time.

I'm building up to fitness level with which to run the marathon in october. Anyhow I am following the 1200calorie plan set out by MFP as I want to loose 20lbs to help my running (lost 5 so far n started 10 days ago!!) Anyhow am I doing enough. SHould I be out more. I do the school run twice a day as well.

Cheers for any responses x