Nice job! Congratulations on your first lucid experience!

Generally it does get easier after the first time, depends on if you stick with the same induction method or try a new one. To keep from waking up immediately, do it more often; you'll get used to it and while you'll still be excited from lucidity, it won't be overwhelming excitement. It doesn't take much excitement to disconnect from the dreamworld; first lucid dreams often are so different an experience that you just wake up from it anyway, excited or not (story of my first WILD right there). Lastly, the position you're sleeping in ought to only affect your dream length if it's uncomfortable after you've fallen asleep for a period of time.