I think the upper limit guideline is 1Km per year, so 2.5K for your little kiddlywink Remember reading it in a book called something like 'how to make your child excellent'.
I would actually not go for 'proper' running trainers, rather plimsoles or something with little cushioning and then make sure he lands on his forefoot (look this up on the net if you don't know what it is).
If he wants to run, let him run, but hold him back at least a bit. Better that he always wants to do more...
Never give in to despair mate. I'm sure you're a fantastic parent (Mum?) in many other ways. If you were perfect, you'd be the first person who was.
Do something to get fit. Don't care what it is. Doesn't matter how many calories, or anything else. Just something that gets you sweaty and out of breath that you'll actually do. If you get bored easily, just keep doing new things, but stick to one thing for a bit. So, you could do your 30 day Shred DVD and have something planned ahead of time, for the next month.