Posted: 17/10/2014 at 18:09
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.
So, in summary - what Eggy said !