OK, time for Running Rodent's Post of Doom:
Obviously everyone on this thread is going to have a fantastic race, smash their PB and generally charge over the finish line in a blaze of glory. But things do happen, and now is a good time to put your contingency planning in place.
1. Carry your EHIC card with you on race day. You're entitled to the same medical treatment as the locals, but you have to be able to prove it, or you'll be billed and have to pay it back. This applies to any medical tests you have as well as treatment, even if you turn out to be fine.
2. Carry a spare metro ticket and some cash in case you have to bale out and find your own way back to the finish/hotel. If you don't use the cash you can spend it on one of the evil smelling sausages from the vans after the finish while you wait for your supporters, although I wouldn't recommend it. Wrap your metro ticket in something waterproof or it'll get sweaty and won't work.
3.Write your emergency contact details on the back of your number. You don't want to be sitting in a medical tent feeling a bit green and trying to remember your nearest and dearest's mobile number, in French.
4. If you have supporters in Paris arrange exactly where you're going to meet at the finish. Pick a landmark, shopfront or particular bench the day before. Don't just say 'the exit to the finish' as there are several and it's a bit chaotic.
5. Have a fallback option if it all goes pear shaped and you lose communication with your supporters. Don't rely on being able to reach them by phone. Phones die, and get lost, or you might just not be able to get a signal. Agree something like 'if we haven't found each other by 3/4/5 o'clock make your way back to the hotel/a particular cafe/bar'.
Once you've got that all sorted, you can relax knowing you have a Plan, and enjoy the race.
To which I'd also add if you're flying into Paris don't put your race day kit in the hold.