I calculated right about 35 miles from 19 to 76 (see below interactive map for distances, elevations, etc.). Only you can answer how long it would take you. I have not hiked the AT but have spent lots of time in the surrounding mountains. They are rugged but the trail hugs the ridge line pretty well. When were you planning on going?
It’s around 37 miles with a fair amount of Topo. I did the 31 miles between Springer Mountain and HW19 in 48 hours and it was too much too soon for my 50 year old knees. With no idea as to your conditioning I can’t tell you how long it should take you. I know if I were to do it again I would limit my mileage to 8 - 10 miles per day and enjoy the trip. Your cardio might be great, but if your joints, ligaments and tendons are not conditioned to doing Thousands of feet of elevation change you will be glad you took it easy. It’s the decsents that will wreek havoc on your knees. You can purchase the guthook app and look at the elevation map to see exactly what you will be facing. The app also shows unofficial tent sites and water sources and has a gps feature that works without cell service and while in airplane mode to conserve the battery. There is a saying on the AT. “The last one to Mt Katahdin wins.” Start slow and enjoy he journey.