Hello cynicalad and welcome to the forum!.
My (non diabetic) partner is a keen cyclist so he talked me into doing the London to Brighton bike ride a few years back and we're thinking of going to Devon for May BH weekend to do (part of!) the tarka trail.
The good news is that the D shouldnt get in the way of your training, but as members have already said, the only way really is to TEST TEST TEST to see how your body reacts to the cycling. For example, in my case if I do any form of exercise when I'm above 10 I need to take 1 unit of QA, otherwise I end up in the high teens, but that may completely different for you. The other thing to think about which Northerner pointed out to me is the time of day you're exercising and what time your last injection was. Again for me if I'm exercising more than 4 hours after injecting I'll end up really high because the insulin has run out, again this might be totally different for you. So there are loads of variables which can affect your sugars (joy!), but the good news is that if you keep a log of everything, and write down any observations you should hopefully be able to see a pattern.
There is also an exercise section here - and I think someone is actually doing the London to Brighton bike ride, so could be worth checking reviewing that as well.
Good luck !
