Hi, The humerus is a nasty injury. I broke mine clean in two on the 2nd July (that's it nailed in my avatar)
For me I had the nail inserted 2 days after the break (strange thing is once the nail was in, I didn't need a cast).
At the end of July I had just about no movement in my arm outwards from the body, limited forwards/backwards and a fixed 50 degree kink.
Got back to work after 5 weeks (which was probably at least week too early - working is significantly more tiring than pottering around at home, so even though I thought I was ok to go back, the first week almost wiped me out and I only have a desk job), and back on the turbo (one handed using a step ladder) the week before.
In the end I was passed fit to cycle again on the 23rd September, so about 12 weeks. The arm is strong enough and flexible enough to do almost everything I could do before, but it is still not fully healed, and I've been advised to stay off the mountain bike for a while yet
You will lose fitness (mine's slowly been coming back over the last few weeks, but hill climbing and resistance against headwinds is still way below what it was in June), but it is possible if everything goes well that your C2C could be on next year.
The biggest bits of advice I will give are to make sure you get physio and lots of it. Mine was NHS, and came in ridiculously short consultations, but if you are prepared to put the work in doing the exercizes they set then this can work. Also as soon as you can, do as much as you can with the arm. In my case I knew if I was overdoing it if the arm felt swollen.
I'll point out that I am not a doctor, and have not seen your injury. Mine was a very clean break and I didn't have any chunks knocked out, which will have helped. I've read reports where it has taken far longer, especially if the arm is braced instead of surgically bolted back together. Don't try to rush back (despite how tempting it becomes). Listen to the medics and I hope you recover soon.
Oh, and at least you've managed to time your break for the cold wet season, I've just had to sit out an almost perfect cycling summer