Hills and Long runs

Hello,
I am currently training for the Paris Marathon in April.
I live in a very hilly part of the UK and it is relatively hard to do my long runs without finding a hill. Running uphill is a great workout. However, I was wondering what the impact of running up and down hills will have on my long runs? Should I try to avoid them in my long run or should I keep them in my training? 
Thanks in advance,
Rod

No harm in having a bit of up and down in there, but might be worth making some practical compromises, e.g. if the schedule says run 20 miles but it's very hilly, don't worry if it's a mile or two short.  Also, Paris is pretty bloomin' flat, with long, lovely stretches of wide, flat boulevards (great for getting into a rythm and zoning out!) but this sort of terrain needs to be trained for as well.  Running at a steady pace over very samey terrain places particular demands on your legs, so you should try to at least include some longish periods of flat running if possible.

+ for PhilPubs advice, also don't worry too much about maintaining pace whilst going up the hills, e.g. if you're trying to run at 10 mins per mile don't worry if this slips to 11 mins per mile etc on the uphill - as its the effort level rather than speed which is key.

As PhilPub said, try and find a flat route for at least one of your long runs (even if it involves driving a short distance) as the change in terrain during training does mean that you are varying the load on your muscles & joints, so they may react differently when they are subjected to a prolonged constant loading.

Thank you PhiPub and Stutyr for your comments. I will try to find somewhere a bit "flatter" for my future long runs. 

Rod