We bought a 20lb'er, wanted to do the high heat method but read we shouldn't, wanted a back up plan, I guess we can give up the oven for several hours

If you're feeling bold, you could spatchcock the turkey, which is a faster method!

http://www.seriouseats.com/2012/11/how-to-spatchcock-cook-turkey-thanksgiving-fast-easy-way-spatchcocked.html

And here's a slow-roasted option if you decide not to go the high-heat route, though it's hard to go wrong with the Barbara Kafka recipe Linsday-Jean just posted! https://food52.com/recipes/31910-slow-roasted-turkey

Barbara Kafka has a guide for a 20 pound bird using the high-heat method, she says unstuffed it will take about 3 1/2 hours: http://www.bkafka.com/Recipes/roastturkey.html