Formaggio - which Garmin are you using? I've used a 110 without a foot pod and it was fine. Watch out though, the distance measuring isn't 100% accurate (tall buildings, weaving around etc). For absolute accuracy you can't beat using your watch for the time and checking it against a pace band at each km marker.
Shouldn't be a problem to have your husband join you towards the end, but he won't be able to cross the finish line with you. They do pull runners without numbers off the course at the last bend, so be prepared to run the last 0.2 miles yourself in a blaze of glory . You won't have any trouble with motivation up the final straight with the crowd howling like banshees...
DJC77 - can't say I have, and looking at the map, none of them looks particularly convenient for the start/finish. Unless it's an especially good deal you'd be as well looking at others nearer the Arc d Triomphe.
1. Suggest you shop around for a marathon plan you can commit to. Some will measure effort by heart rate, others, including my own personal favourite P&D (Pfitzinger & Douglas) use speed (eg '3 x 1,600m @ 5k race pace'). I've never used an HRM, but others here have had good results.
2. If Lucozade and jelly babies work for you, then stick with them.
3. The GPS should pick up again once you're through the tunnels, but even then you could be up to half a mile short by the end of the race. Safer to use a pace band (get one from the Expo) and make sure you're hitting your times at each km marker.
4. You can forecast your times from the McMillan running website, or the McRun app. They suggest 4:10-4:15 off those 5-10k times. Although as you get into training your shorter distance times will come down so you may be able to adjust your target.
Yes, I fled south nearly 4 years ago - I don't think the police have caught up with me yet . The Meadows was the site of my first ever 'run' a thousand years ago - 20 steps walking, 20 running. I was so pleased to make it round I never looked back .