As a beginner to jumping, put distances aside for a bit. They will come with time, practice and mostly feel - in that, to find a good distance, you must feel the canter stride and project it to the base of the next fence, in order to know if you need to shorten the stride or lengthen the stride to hit the fence just perfect.
When starting out, you want to work on balance, proper release techniques, keeping heels down, leg stationary, keeping eyes forward over the fence, chin up - in other words, master your position first. Gymnastics can really help with this (long lines with 5 or more fences consisting of bounces, 1 strides and 2 stride fences). And working without stirrups too.
Now to work on feeling the strides of your horse. Do this first with no jumps. Drop the fences of say a 6 stride line so its just a single pole on the ground for each “fence”. Canter your corner to the first pole, and when the horse clears it with his hind legs, start counting your strides. When you get to 6, your horses front legs should be in front of the second pole - if he’s too far away, you need to lengthen your canter - if you’ve passed the pole already, you need to shorten the stride. You want the canter stride to both poles to end up with the pole directly under the middle of the horse in his stride. If you’re reaching the poles where the horses front hooves land right in front of the pole or right after the pole (like within a foot), some lengthening or shortening is needed. You want the pole right under his tummy.
Distance is all about lengthening and shortening the stride. Just keep practicing with poles on the ground. The horse won’t jump the poles mind you, they are just placeholders for where the fences are. Release like they were fences. The more you ride the canter and practice lengthening and shortening the stride, and really feeling the ground you are covering, the more “seeing the distance” to any fence will become second nature.
As to stride length, the average horse has a 12 foot canter stride. Some horses are less, some horses are more (and some a lot more - my 17.2 Draft cross covers significantly more ground than 12’ in a medium canter stride). Learn to “walk a course” and measure the strides between fences. An average height woman taking fairly large steps covers 3 feet per step, so 4 steps is 12 feet. So every 4 steps is an average stride between two fences. Put a yard stick down or tape measure on the ground and practice what a 3’ step feels like. Walking a course and measuring the distance can really help you get a feel for the proper canter stride length than someone just telling you “its a long 5 stride” or “its a short 7 strides”.
Good luck and welcome to the wonderful world of jumping!