There are 2 main factors that need to be remembered prior to any C2C trip :-

i. make sure you are physically fit
ii. make sure your bike is mechanically sound

You do not have to be a superb athlete to complete the C2C; the key is to plan your route so that you are capable of completing the necessary miles for that day. Don’t be too generous when planning how many miles you expect to complete each day. An important thing to remember is that you may be carrying all your equipment with you in panniers or rucksacks. You might not be used to riding with all this extra weight, just because you can competently ride 50 miles a day at home does not necessarily mean you can ride 50 hilly miles complete with luggage.

Similarly you want to make sure your bike will not let you down, I make sure that my bike gets serviced by my local bike shop each year (you have your car serviced so why not your bike). However if this seems overkill as a minimum you make sure of the following :-

Brakes should be in full working order, particular attention should be quality of blocks.

Cables for both gears and brakes should not be showing signs of fraying.

Tyres should have adequate tread on them & be correctly inflated.

The saddle should be comfortable and not showing signs of tears.

All mechanical parts of the bike should be sufficiently lubricated.