I have type 1, use a pump & enjoy cycling, am nowhere near as fit as you but do it pretty often. To give an idea of intensity, when I ride with my kids I run at about 60-70% of my max heart rate & if riding alone I'm above 70%, with decent periods of 80-90% of max (based on my age). I do between 35min & 2hrs. My BG drops fast in the first 30min, then usually stabilises & rises after I finish. I try to manage it by altering my insulin rather than using carbs, though I take 10-15g fast acting carbs 20min before I start if my BG is under 6 & often need around 10g if it's down to under 5 after the first 30min.
You essentially have 3 options: reduce your basal insulin (harder on injections than a pump - you'd be a very good candidate for a pump), reduce your bolus if you plan to exercise within 1.5-2hrs after eating, or take in extra carbs, or a combination of these.
Very intense exercise can usually raise your glucose levels, moderately intense exercise (I think 70-85% MHR) drops it fast & moderate intensity exercise reduces it more gradually & by less.
Runsweet is an excellent website with loads of information about diabetes & exercise, backed up by research. It's contributed to by a diabetes consultant who's done loads of work on this area.
Basically you'll need to test lots, record what happens every time & learn through trial & error to some extent. I don't know if you can join at 13 but there's a Facebook group called Sporty Diabetic Type 1s & people on there do Ironmans & there's a cycling team called Team BG made up of Type 1s.
It is tricky, but not impossible - good luck!