There are lots of posts about how to do this, but I would test 1 and 2 hours after every meal to see how it affects.your bloods
If you intend to do energy drinks still I would test after those also, you may be surprised how much they hit you.
As a note how long does a 70km bike ride take you to do? I do quite allot of excersise and I don't do any carb loading, I do eat before now but I find something while Wheaty and carby gives me all the energy I need and it's a slower burn. Two shredded wheat is like 60g of.csrbs I think but it takes way longer to digest so I find it doesn't hit my bloods, if I drank 60g of carbs in sugary water I would see a big spike quickly.