So I just put SPD clipless pedals on my hybrid. I have been averaging around 20-30 miles with no sweat on my hybrid with flat pedals

Today i was trying to go for a long ride with the new pedals, however after about 12 miles my legs were toast! is it normal when you make the switch for the legs to tire faster? and if so what can i do to increase my stamina on the new pedals?!

(no problems clipping in and out fell a few times no biggie, i got it now)

I've experienced the same thing since I recently installed clipless pedals on my road bike.

IDK, you find you use a higher gear than you did before , to use the firmer connection,
and unintentionally putting in more work effort out?

Where is it getting sore?
I'm guessing your cleats might need to be adjusted. Or possibly your leg reach to your pedal has changed with the new setup and he saddle needs adjusted.

No I didn't find that to be normal. You might not have the float you need and are getting used to keeping your feet in one position. It your cleats are too far forward or if your toes are pointed in with your heal out to too far that will tire you leg. And as someone has said if you start pushing a bigger gear because it is easier to do it could tire your legs.

Have not heard this before. I have heard about feet hurting as people adjust to the new "foot position" on the pedals, but never legs.

First thing you should check is whether you are in a good position with the clipless pedals. It is possible they affect your riding position as you previously may have pedaled with a different part of your foot. What you could do it check this and adjust bike setting accordingly. But you could also adjust the cleats to be more comfortable.

With bieks, any adjustment to you motion/position/posture can cause discomfort, so it does not surprise me. Take some time to try different setting and see how things pan out.

I moved the cleats back a little as i noticed i was pointing my toe an awful lot, and i think you are right I was pushing some bigger gears Because It was easier to do so but I still had less speed than i did at smaller gears on my flats

It might just have been an off day I am gonna attempt again tomm, But i also think its because my feet arent used to staying straight on the pedal, how can i adjust the cleat so they arent quite straight