There are two groups in England
EGB who are the biggest organisation and are FEI registered
Sport Endurance who are a smaller group they are not yet FEI registered
Both groups have websites with information on them.
I am a member of both, I joined SE first as they were cheaper (to see if I like it) and had lots of local events. They are very friendly and held training events that taught you about vettings etc. As my horse got better and we became blue level with SE (top level) I joined EGB as well as I have aspirations to compete at 100 mile events and needed more access to 50 mile/80km rides that SE don't hold very often.
I have 2 very good books also 'Go the distance' and 'Endurance Riding: From First Steps to 100 Miles' The second book being my prefered one
You don't have to join to compete at the lower levels, the smallest distance to compete in is 20miles/32km.
Best way to start? Long hacks, as with all fittening regimes build it up slowly and keep a training log
For a totally unfit horse
Start with lots of walking building upto an hour 5 days a week. Then introduce trot 10 mins total out of the hour, build up the work to 1 and half hours. Once ok with this increase trot to 20 minutes of total, build the work up 105mins, then to 2 hours with 30mins trot
Then you change to something like this or start here if fit.
Day 1: 2 and half hours at good pace, Day 2: 1 hour gently, Day 3:rest, Day 4: 30/40mins lunge, Day 5: 1 hour schooling, Day 6: fast work or pop over jumps or hack out
Day 7 rest
Take advantage of hill work tot/canter up, walk/tot down
Before first comp, practice a 15-20mile ride, you need to average 6-7mph, so a steady trot is best (remember to keep changing diagonals)
After do a mock vetting 30 minutes after finishing: take pulse (needs to be under 64) and the trot up.
If you want any more info give us a shout
Sorry for the essay I love endurance