OP probably means Kalaw--which to be honest, is pretty clearly not connected by any direct route to Hpa'an just by looking at a map. Yes, buses always stop in Meiktila, but at a big rest stop not a bus station. From Meiktila station there may be a few standard buses going on the eastern road towards Taunggyi, but if you can't get a seat on one there will be linecars (very uncomfortable) and possibly minibuses.
I would ask around first if there are standard buses directly to Taunggyi (this is not a thing that existed for foreigners when I visited Hpa'an, but things are changing rapidly), and then from there take the linecar/minibus/whatever to Kalaw. I've traveled from Taunggyi to Loikaw (along the road between Hpa'an and Taunggyi) but it was a cramped minibus and pretty slow...if that's still the only way you will probably still be better off going the central route.
You can do this in less than 24 hours, but it may not actually be a single day (because of night buses).