This is the way. I’ve been using an older Samsung phone for this purpose, and not only does it offer fitness tracking, but the navigation options are far better than anything a bike computer could offer, since there are dozens of apps available to suit every need.

There are caveats with a phone, though. Battery life isn’t as good as a bike computer; they are not easy to read in the sun; they aren’t quite weather resistant, especially in sub-zero temps (mine gives charging errors or shuts down when it’s too cold).