Testing is the way to go. We can give you lots of tips about what works for us (and I'll give mine below) but ultimately everybody's diabetes is different, and you'll have to find what mostly works for you. That means trial and error, so if you plan for the worst, you'll be all right. Take more food than you think you might need (and/or money to buy more) as well as a source of fast-acting carbs (like glucose tablets).
Take a few notes to record what you do, such as changes in basal rates (if you're on a pump) and how much you ate and how long you rode. Then tweak it until *you* are happy with it.
Here's what mostly works for me now. When I ride in the morning--which is easiest for me--I lower my basal to about 60% of normal about 1-1.5 hours before I head out. Then about 15 minutes before the ride I eat something with a lot of protein and about 20g of carbs (yoghurt). During the ride, I eat about 20-30g of carb (gel/banana/Clif Bar/etc.) every 30-40 minutes. I don't usually bolus for any of the food before or during the ride, and this keeps me pretty stable.
And then I need some insulin right after I finish a ride to keep my BG from shooting way up. That's me.
As for CGM, it's great most of the time, but I find mine to be rather unreliable when I'm on the bike. So just keep in mind that it's not a substitute for testing when you're riding.
And most importantly, HAVE FUN!!
I'm right there with Jeff insofar as recommendations are concerned up until his finding his CGM "rather unreliable" during rides. I'm just fine with the numbers my Dexcom feeds out during rides, and I react accordingly. Best part for me is I have the low alarm set such that when I hear it or the downward trend alarm go off, I don't even have to get off the bike, but just grab the chomps or shot blocks in my pocket, down a few and keep rolling. If I am feeling tweaked, I will certainly stop for a few minutes, but I don't always have to do that.
Good luck out there.
My CGM is getting more reliable as time goes on, but it's not good enough (for me) to use it as my safety net in quite the same way I do the rest of the day. It doesn't catch a lot of fast drops (for me).
But I do love having it! When things are going well, it's nice to look back over the last few hours of riding and say, "That's a really great line!" or "It was probably about then when I started trending up/down. What can I do differently next time?"
I acknowledge my CGM may be off due to dehydration, and usually lags behind in accuracy, but I wouldn't ride without one any more.
Watching your HR will be a good benchmark too. Again for me, the higher the HR, the faster the drop in sugar, UNTIL!!!! I hit my HeartRate max. 210-33(age), or about 177. Then it goes up. So on climbs, my HR will be about 185-190.
I set my basal rate at 70% for a 1 hour ride, 50% for a 2 hour ride, and 25% for longer. That is usually too low, but I watch the CGM and HR monitor. And a gatorade for every 30 to 45 minutes, and a carb bar (berry blast powerbar is my favorite) for every hour.
My target BG is 150-ish for starting any activity. I let the activity and availability of carbs set the BG after that.
Congratulations on getting back on your bike! I quit for about a year after my diagnosis (I was severely depressed and having trouble coping). Getting back on my bike was one of the best things I did for myself. Shortly there after I also participated in a Tour de Cure event, which was a ton of fun. I found that my overall basal requirements decreased by about 15% when I started riding regularly. Keep testing as often as you can so you know how _you_ respond. I ride with a CGM. I did fine without it for 2 years, but I've found it takes the anxiety about low BGs out of the equation. I agree with miketosh, the accuracy varies depending on a lot of conditions, but I like knowing that my BG is changing rapidly, or is steady before I head into a long climb.
As far as dropping BG and biking goes, I find that biking drops my BG more than any other sport I engage in. I have to really pay attention to the amount of insulin I take for meals before a ride, and to fuel properly before and during. When I first started riding I came across this post: http://www.tudiabetes.org/profiles/blogs/proven-formula-rough-draft which I found very helpful. Now I'm training with TeamWILD, and the author of the above formula is one of my nutrition/diabetes coaches through them. Her help has been invaluable. The more information we can have, and know about diabetes and exercise the better off we are. You can also check out TeamWILD (We Inspire Life with Diabetes) at http://teamwildathletics.com/.
Test, test, test, be safe, and above all, have fun!