Two thoughts - one about what I do when I prepare to ride alone, and one about not preparing and trusting that nothing will happen....
I condition alone all the time. First, I make sure my horse is in the right sense of mind to be going out on his own - misbehavior or a particularly spooky and flighty day means we're conditioning in the arena while working on discipline. You'd be surprised how much conditioning gets done just by regular riding in the round pen and arena, especially at a trot and lope! I carried my GPS on me one day for an hour, and we covered 8 miles.
When we do go out, I generally let someone know at the barn where I board that I'm going. I also remind them about a cool little map I have hanging up with my tack in the tack room: I drew a map of all the trails that I ride and that are known to the other people at the barn and the owner. Then, I laminated the map, hung it up, and hung up an old sock with a dry erase marker inside. When I leave, I mark the trail I went on with the dry erase marker along with the date, time I left, and time that I plan on being back. I'm going to add an emergency number as well. Then, if I don't come back, or if my horse comes back without me, they know where I went and when I was planning on being back so they can send someone out to go with me. If the horse comes back without me, even better - I keep my GPS on the horse and they can use that to track down where I went. Of course, I also ALWAYS have my cell phone on me - not my horse. This is a little difficult to do with riding tights and without a jacket, but I've found that my phone fits nice and snug in my half chaps. I've got a pack that wraps around my leg and has a spot for a cell phone and other emergency equipment in the mail. That way, I never have to worry about my stuff running away with the horse. Finally, if I am by myself (which I usually am), I stick to trails that a 4-wheeler and usually a truck could come down - if I am badly injured and need rescued, I would prefer to be rescued by a vehicle lol. This isn't possible in a lot of areas, but I'm riding on BLM land with dirt access roads, so those are what I stick to.
Now, about a lack of preparation - you NEVER know what will happen, especially on trails. You can have ridden your entire life and have a horse that you've ridden for years and is "100%" trustworthy - and then a rattlesnake pops up under it's feet and off you go, or any other unpredictable incident. Even a helmet isn't always enough - that won't prevent from a wild animal bite or breaking your neck. Being endurance riders, this one will also particularly hit home - I remember QOS talking about a ride she went to that is dedicated to a very experienced rider who was wife and mother riding a horse she's ridden for years and was very well trained. A freak accident happened and she went off her horse. The fall isn't what killed her - the horse's hoof hitting her face on the ground did.
You never know. Working with horses at all is a dangerous sport - much less taking them out into the "wild" all alone and riding at high speeds in one of the most extreme equine sports. I think most of us can admit that we don't plan to get in car accidents, yet cars are equipped with countless safety features "just in case" - and the cars don't even think for themselves like horses do!