It takes about 10 miles for my 1997 heater to start blowing out hot air. Is there a valve that controls the water going through the heater core not opening? The temperature leaver is on the full hot setting

As in every car, the heater just needs to warm up. If the temperature gauge in your car registers between 25%-60% heat then your heater will work fine. One can't expect a heater to be hot right when they start their car, this is the purpose of warming up your car before you drive it.