Hi Kathryn
I could fill page after page with information about endurance riding but I give you some easy starters.
For some general information about Endurance riding read
http://www.ehps.org.uk/home.html
For a training scheme for novice horses (and riders) read:
http://www.horsetalk.co.nz/nzendurance/guide.htm
I see you are scotland based so check out the Scottish Endurance Riding Club:
http://www.cee.hw.ac.uk/~rose/serc.htm
If you have read them all read also:
http://www.endurance.net/
http://www.british-endurance.org.uk/
Arab horses are THE endurance breed, but many breeds can perform quite well when trained properly and you don't need to be a world class rider.
My personal choice would be an Anglo Arab, because I am tall and a little bit of a heavyweight and would like to have some more height and bodyweight in the horse itself than the average arab.
I don't ride endurance myself (yet) but have friends who do and I have attended and groomed several rides of them, and I think it is a wonderful discipline with a great atmosphere among the riders.
I woul suggest you read a book about endurance riding e.g. from Clare Wild (UK) or Nancy Loving (US) to know a little about it. The best thing is to do it yourself and learn from the more experienced riders, they tend to be very helpfull to each other and to novice riders.
It is the marathon of horse riding and it asks quite a lot of horse and rider, and years of training to reach the levels of 50 miles and over, but I love it.
Good luck and enjoy it
Alwin