Howdy Captain!
So as I understand it, you're pushed for time and injury prone, but want to run and lose some weigh - maybe going on to do some races...
Right well firstly - you should be able to squeeze in a run
somewhere in your day... even if it's only 30mins. I always find that a nice run destresses me no-end so is a great way to help you through the working week... even if it's just a short run - and whilst I can imagine it being great to run across the downs, there's nowt wrong with just popping your trainers on and having a run down the road or round the estate (depending on your location).
As for training content - as long as you don't push hard from the outset, you should be fine to run the entire distance rather than walking (if you feel the walking isn't exercising you enough). There's plenty of training schedules around for people of all abilities - so it's just a case of searching for one you think you can manage and just going for it...
I hope that's of some use, and not just complete waffle! But feel free to ask as many questions as you can think of and the forum shall help all it can
Oh, and don't worry about your weight... it'll soon drop once you're running
