Re: sponsered ride distance to aim for
All the best with the ride as it is for such a good course.
It is hard to organize many endurance rides and im also doing a ride myself in June for Help for Heros, the hardest part is finding land owners and getting permision to get across there land if you can use as much bridleways as you can then this will be better im doing mine from Longleat going to the New Forest around the New Forest then back again (200 miles give and take a few miles) as for training i would try and do a few Endurance rides starting at around 25miles working upto 30 miles not only will this tell you if your horse is fit enough but also if they could cope as you say you like to do it in 3/4 days you dont have to be a member to do there rides.
As for followers you will need to work out point along the way for your mum to meet you to give your horse water feed and the same for you again this should really be done every 8 miles MAX this will also give you the chance to make sure your horse does not have any sores or is lame.
FInding yards will be your next problem the other thing is to get in contact with farmers to see if you could borrow a field for the night and corral your horse (just make sure its ok with elec fencing) and if its ok for you to camp. Depends on where you are doing the ride if you are doing it on a very well known area for Endurance then there be stables aound but normaly when we do big rides we either stable at the venue or corall the horses (if you like help with this then feel free to pm me as i can get you address)
Feeding keep to your normal diet and if you dont feed it already get them used to Speedi beet as this is a very good drinking water as well as electrolites, I personly feed just a plain fibre diet my 100milers get Dengie Hi Fi and Dengie Alfa A Oil with speedi beet and i find this gives them enough energy to do 100 mile rides. You know your horse so i would really feed what you normal feed.
Training: I get all my endurnace horses 100 mile fit by doing A LOT OF WALKING this is very important for legs and mucsles the only time i gallop my horse is when we are racing (endurance racing) i know this sounds boring but walking is the most importent thing and i swear this is how i have kept my 25yr old going and doing high millage endurance (still going) but then you would know this.......sorry.
I would do what you are doing building up slowly if you try and take short cuts or rush then you will not mannage the 100 miles remember to get in lots of hill work again walking is better then trot and canter as they use them selfs a lot more and its harder for them, if you dont have access to lots of off road riding where you can get a min of 10mile rides in (which should take you around hour to hour and 15mins to do) then you could do interfull training but im not a fan of this as ive seen to many horse break down.
You have plenty of time and if you have started now then you SHOULD be fine but i would really try and get some endurance rides in as you get a fetting and you can do the milage.
If you need anymore help feel free to pm me