Definitely don't do it. Your training is done! I've done a few marathons... all you can do is harm at this point!
If you are worried about weevers you just need some aqua shoes/wetsuit boots or something to walk into the lake. You could even use cheap waterproof sandals and chuck them back to land once you're in!
It will probably be fine though - I bet most people will go in with bare feet.
Have you been to a doctor? It could be a stress fracture - you should get it checked out.
I'm in the same position WeV, just started to get back into running after 18 months of zero motivation following a period of depression. A few things have helped me get back:
1. find a friend to run with, someone who is close to your pace or who has signed up to the same event as you.
2. Don't be too hard on yourself - start with running once or twice a week, then build up to your old schedule.
3. Sign up for a few shorter events building up to your marathon, then it won't seem like such a big mountain to climb.
4. Try and mix one or two other activities in with your running - something you could continue if the worst happens and you get injured :-/
Good luck, and stop being so hard on yourself... one day at a time
Are you sure it couldn't be asthma? A trip to the doctors to figure out if it's asthma or allergies wouldn't go amiss
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