Hi Naffets,
I did the CtC on the weekend of 6th June with 37 friends & colleagues - 7 of which did it in one day!.
If you are looking at one day, you need realistically a roadie or cross bike and need to get in long training runs, at least a couple of 80 miles or so with some monster hills.
You also need to take advice to NOT take all of the Sustrans route as you can shave 15 miles plus off your ride as well as a few unnecessary hills (for seasoned cyclists and commuters not afraid of a little traffic).
Hartside is doable but nowhere near as bad as after that across the pennines.
Starting with the climb away from Alston over Nenthead to Allenheads then a lovely (!) climb out and over the highest point opf the pennines across to Stanhope.
Out of Stanhope is a vicious 2.5 mile climb "Crawleyside Bank" which will make you want to weep and wonder why in hell you do this sort of thing.
Once at the top you get to Parkhead Cafe and the bestest ever bacon sarnies before your almost 40 mile downhill & flat to the finish at Tynemouth.
We did it in two days (myself & 29 others) and even though I had done a few 40/50 mile training runs with some lovely roads and hills between Newcastle & Hexham, I was still not prepared for the CTC - That said, I would do it again but on a mountain bike with proper hill gears (I did it on a Hybrid).
What didn't help us was that we had a weekend of 15mph+ headwinds gusting up to 30mph ad it rained all Saturday morning and Sunday morning. In other words, the weather & wind were SH*T.
Be prepared to lose your sanity at some point - I reverted to verbally abusing sheep after being bleated at when a nasty gust of wind stopped me dead in my tracks half a mile from the top of Hartside at 7.45pm on the Saturday!
Only 3 2-day riders pulled out - two because it was too much and one who's knee went.
Also one of the one dayers due to another knee injury.
It was a F*&king great experience though.
Also, take good advice on nuitritional build up prior to the run and build your fluid levels up well in advance an on the run.
But mainly - enjoy it.