Map Assistance

As I mentioned recently, I'm trying to put together a map for Secrets of sha-Arthan. I've already got a continental outline for the whole world, but my focus at the moment is on a smaller region intended as a "starting area" for new characters and campaigns. Nevertheless, I have several problems and I'm hoping readers might be able to point me in the right direction for solving it.

Here's a portion of the continental scale map of sha-Arthan:

Because it's continental scale, those hexes are very large in size – about 90 miles across. That size is based on two factors: that sha-Arthan is roughly the same size as Earth and the number of hexes used in the continental map. I could, I suppose, redraw the continental outlines and then layer on a small hex grid in order to make each hex smaller, but that's something I can't easily do, given my lack of technical skills. If someone can suggest a relatively easy way to do this, so easy that even an incompetent like myself could do it, most of my problems go away.
Barring that solution, I was intending to break down each 90-mile hex like so: Done this way, each of the sub-hexes within the 90-mile hexes would be 18 miles across. These 18-mile hexes would be further subdivided into 3.6-mile hexes. Both 18 and 3.6 miles are odd increments, to be sure, but that's what happens if I stick to 90-mile continental hexes. A possible "solution" is to create in-game units of measurements that correspond to 3.6, 18, and 90 miles. On some level, I like that, because it's potentially immersive, but it's also potentially annoying and easy to forget. 
Another possibility is to use a different set of divisions for each continental hex. The only reason I went with the five hexes within five hexes set-up above is because I was able to find some templates online. I'm not kidding when I say I am utterly incompetent when it comes to these matters, so it may well be that there are simpler configurations that will serve my purpose – that purpose being breaking down continental scale hexes into small, more manageable ones for use in play. 
Any ideas or suggestions you have to share would be appreciated.