no idea on the condition of your horse or the conditions of the ride you are attending. You can judge based on heartrate, you need to get out and ride 15 miles at a 7-9mph pace minimum. when he can do that and pulse down to 60 in 15 minutes or less he's probably ready for a LD ( limited distance) actual endurance is 50 + miles . Ride under those trail and weather conditions. Keep in mind warmer temps and tougher terrain can change that. This is pretty much where I am after 9 months of training a TWH,,, obviously not an ideal breed so needs more work. And I still have to pick my rides and wont be doing any over 80 degrees. Hoping as condition improves we can move up in distance and speed.
The thing you also need to realize is Cardio gets in shape first, but bones, ligaments and tendons last. So rushing into it can get you a horse that pulses down, but comes up lame . Takes time to build up connecting tissues to the rigors of all day trotting, loping,, or run/walk and rack.
So to answer your question, it really depends on what kinda shape the horse is in now, what breed and build, and what the terrain and weather will be like on your chosen ride.
I strongly sugest geting the book endurance 101 you can download a kindle version. really good lighthearted easy to read book. Welcome to the sport, now yippy kai ay, Let's ride !