Swimming and cycling don't place the same stresses and impact on your legs and so that would help. They are often recommended as cross training exercises to keep up fitness if you can't run (would depend on the injury though I suppose).
Correct shoes and not over training are the biggest things you can do to help yourself avoid injury I believe.
I'm not sure how much help this will be but regarding your stride I'd say just do what feels comfortable. For me, artificially shortening or lengthening my stride is difficult, there is a natural stride (up hill the natural stride, for me, would be shorter than on the flat). Just see what comes naturally.
I'd put a time limit out of your mind, personally I think 80 miles a week in 6 months is unrealistic. Depends how fast you run as well, if you were a 10 minute miler for example then you'd be running quite a few hours a week to get to cover this many miles. I believe it takes around 12 months for your ligaments and tendons to get stronger as well.
Too much too soon and you'll get injured and disheartened. If I were you I follow one of 0-5k programmes on here, build from there and most importantly enjoy it
If it's really a chest infection then don't run whilst you're ill, bad idea. i did a 10k last year against my better judgement and I was really ill for a few days after. Zinc and vit c supplements really help me.
I've been stopped a couple of times, it's annoying but I've just bobbed up and down on the spot at the same time. Someone once stopped me when it was absolutely throwing it down and they had a nice big umbrella . I just had water dripping from the end of my nose...
The most bemusing incident was when a women out walking stopped me to ask where I got my flashing arm band from as she wanted one for her dog!