Before Hunny had her baby there was only one horse I liked to ride with for conditioning! She is a paso mare and her and Hunny match each others steps almost perfectly when gaiting.
Besides that I prefer to go alone for the sake that I can do what I want.
There are cell phone holders that you can buy that just snap to your boot or slide onto your arm. My jods have zipped pockets in them, so I keep my phone in there. Make sure you always have a good knife on you in case your horse gets stuck in something! Also make sure you have enough water and a power bar that is HIGH IN PROTEIN AND LOW IN SUGAR as this will give you the boost you need! Sugar dehydrates you, protein feuls you. Try to stay away from carbs as well, they will make you feel sluggish and more hungry then not. Protein will fill you up. My favorite power bars are Quest Bars in the Cinnamon Bun flavor! They are AMAZING they taste EXACTLY like a cinnamon bun...its kinda crazy...! And they are good for actual endurance rides as well. They have enough protein, fiber, and vitamins in them that they actually can replace a meal quite easily.
I also keep a caribeaner with me just in case a ring on my saddle breaks and I need to reattach a breast plate or bags for a quick fix til I get home and can replace the broken part.
Ummm...I also keep advil in case I get thrown and can quick take a couple to reduce any swelling I may feel coming on; also a good headache help. But make sure you can identify between a little ouch headache and a concussion so you don't do more harm then good.
That's bout all I got :)