If you can do 20 now you can easy do 26 on a few hills by October! I did Chester last year and a bit hilly but not too bad and a great first marathon experience.
I wouldn't have thought Bournemouth was very hilly as a lot of the route is along seafront but unless you want a weekend away I would choose a more local marathon as that takes away all the worry about travelling and getting back home when you are tired. (I did Chester as I was visiting family)
For anyone in South Devon who fancies a nice 3 mile off-road run on a Saturday morning there is a new parkrun starting this Saturday on April 5th. It is based at the National Trust Parke estate just outside Bovey Tracey, just a few minutes off the A38. Lovely run by the river and free parking and the usual café, toilets etc handy.
In case you don't know parkruns are free and you can just turn up and run. If you want your time recorded then you can register at the parkrun website and download a barcode which is scanned at the end of the run. Everyone welcome and children too (although younger ones need to run with an adult).
I am doing the marathon - if you look at the website you will see it is organised by a couple who have done 100's of marathons and organise loads so I am sure it is going to be very well set up. Have had an email from them with pre-race details and one of the plus points for me was that it was a smaller, low key event that the big ones.
Somewhere on here the organiser - Dave Major - posted some info too - can't remember the thread name. I have also emailed him a few times about local accommodation etc so sure he would answer your queries.