Lol I can take this mare on a 12 mile ride and all she wants to do is go. I can have her canter up steep mountains (not down lol) and after I think she has had enough and we turn to walk home she wants to keep running! Lol anything under 12 miles is easy for her. I have been starting slow with her sense she sat for the summer but we are also fighting her being barn sour -_-'. There is a large hill by where I board and I have been totting her up in and making her walk down it slowly.
Well, the "we turn to walk home she wants to keep running" is not uncommon with many horses and (unfortunately) the norm for barn sour horses. It's not a sign of them wanting to keep going at a demanding pace
. They can be pretty tired and still willing to use whatever energy they still have to get home as quickly as they can. For my money it's not a good habit and can be a tough one to break.
Eole pretty much covered a good starting program. Especially for an 18 year old that's not already doing demanding riding. Even when you want to build up the speed don't push it do much. In both cases you increase the distances in stages. I'd say wait at least a week before moving up (longer if the horse shows signs of tiring or slower recovery).
I'll add to what Eole said and say that while you're doing the slow, long distance riding you can increase the amount of weight you're carrying by a few lbs (don't get carried away). In the short run is won't do much, but over time the low level stress has the effect of increasing the size and density of the cannon bones. From the pictures you already look to have a bit more size than I see on most of the Arabians I come across which is a good thing (you shouldn't have as far to go
). And as was already stated, watch your horses heart rate, respiration and temperature. They are the best (and easiest to overlook) indicators of the condition of you horse since it's not always obvious to see.