You can develop an "entry" strategy based on economics. If you don't mind purchasing new and either reselling or keeping as backup then you might consider a hybrid which could wind up being the "right" bike, but if you get the bug to go faster/further (and you can certainly do so in your 50's), then you will yearn for a road bike. Used would make a lot of sense given that most of your initial riding will be an assessment our the enjoyment you get out of the experience and your butt and brain won't perceive that much of a difference between a road, hybrid or mountain bike over short distances. There is no wrong way to get started: see bike, buy bike, ride bike.