I'll prefix my reply by saying that I'm very much in the minimalist shoes / pose technique of running these days, and have been moving that way ever since reading Gordon Piries book in 2001.
As always Jim if you're getting proper bad pains/injury symptons best to visit a physio or doc for a checkup rather than listen to idiots like me online.
I sometimes get shin pains when running or walking, it is normally because I have suddenly upped the intensity or amount of my training too quickly. Remember that your muscles develop much faster than your bones, so when upping your training your muscles and CV system will be ready for more miles (or intensity) before your bones are.
I sometimes also get numb feet, normally because I've tied my laces too tight. Are you tying your laces too tight to keep your shoes on because they are old and falling apart?
Shoeswise - when my shoes have no tread left or are starting to fall apart, that's when I replace them. For me this normally happens within a year or so (so definitely within 3 years). I ignore the 500mile limit these days as the shoes I wear have next to no cushioning in them anyway, it's also convenient that the folks selling you the shoes say you should replace them every 6 months too isn't it...
If you run with good technique Brooks green silence should be great shoes Jim, I am currently doing all the running that I do on roads in these:http://www.shopnewbalance.co.uk/RC130XLU
find the edge of the hurt envelope - and push...