Oh dear - in a way I'm sorry that you've made that decision but glad if it helped. It is surprising how long it takes for even small dogs to fully mature. Lots of years of shared walks to come
It's OK - some of websites I looked at suggested a dog should be 18 months old before introducing them into hiking / extended periods of exercise.
He's pretty happy with doing 5 miles so we'll keep to those distances for a few more months then build him up to longer walks in the new year.