I am new here and would like some advice. I have a Specialized Carmel 26 (21 spd) that I am turning into a single speed bike. Why you ask? I have back problems that allow me to actually ride only this bike because of comfort. Also I have a toddler (boy) at home so my riding is hit and miss. I have done 25 and 36 mile rides on this bike, which people say is alot for this bike. I have rode a single speed bike and love it. I want this bike turned into a ss for less maintenance and simplicity. On a ss bike if I want to go fast I will have to peddle faster. My local bike shop is going to do it. They have not done a ss conversion on this bike before but do have the conversion kit. Cost will depend on if it is a threaded hub or freewheel hub, any advice before I start?

You mean cassette hub and freewheel hub. Freewheels use threaded hubs.

have fun