Thanks for the comments. I'll have to give it some serious thought. I've done 2 hour long works this week, but my heart rate is only averaging 114. It needs to be above 117 at least to do some good, though I guess some exercise is better than none!
I've been trying to start running for 2 years now, but other commitments seem to get in the way, commuting, family etc. The best I've acheived is a 1 hour brisk (somethimes very brisk) walk over lunch time 3 days a week. Anyone got any tips on how to progress from here?