My last post disappeared, so I'll try again. If you're traveling from Edinburgh to Skye, count on six hours, and more if you need food or toilet breaks. Skye is really busy these days, as it has become a major tourist destination. Given that, please make sure that you have pre-booked accommodation before you go there. You can always find last minute accommodation in May, but you may end up way out in the hinterlands, and nowhere near any restaurants or shops. If you get in to Portree early enough, there is a cluster of B&Bs along Viewfield Road (A87), just before you get in to town. I often tell people to try the Dunollie Hotel in Broadford if you can't find anything else. It's not the Ritz, but it'll do for a night.
My recommendation for your itinerary would be to cross the Forth Road Bridge, and take the M90 toward Perth. As you approach Perth, be very careful. It's really easy to get on the eastbound portion of the M90, and end up in Dundee. I've done it! Go west on the M90 around Perth, and pick up the A9, going toward Pitlochry.
Go straight on through Pitlochry, which is a charming little town, and stay on the A9 to Dalwhinnie. Go left on to the A889, and follow it to where it ends at the A86. Turn left on the A86, which becomes the A82 at Spean Bridge, and follow it in to Fort William. Once you reach Fort William, just before you come in to town, you'll want to turn west on the A830, which at one time was known as the "Road to the Isles." Back in the 1970s, when the 830 was a single track road with passing places, it was a nightmare. It's now vastly improved.
At the end of the A830, you'll come to the small fishing town of Mallaig. From here, you can catch the half hour CalMac ferry to Armadale on Skye. There are nine crossings a day, but it would still be a good idea to book ahead. There are several restaurants and two supermarkets in Mallaig, so you can stock up while you wait for the ferry. The toilets in the CalMac booking office are the cleanest in Mallaig (usually!).
Once you arrive on Skye, you can make the decision as to whether you want to stay overnight. There is a lot to see, and if you are the outdoors sort, many hikes to take. Most of the shops are in Portree, and there is a large (free) car park at the bottom of the hill as you come in to town.
As you leave Skye, you'll follow the A87 back through Broadford, and on to Kyleakin. From there, cross the Skye Bridge, and head east. You'll pass Eilean Donan Castle on the way. Stay on the A87 to Invergarry, then turn south on the A82 toward Fort William. Go through Fort William, then follow the A82 south toward Glencoe.
After you leave Glencoe, stay on the A82 to Crainlarich. You then have two choices to get back to Edinburgh. The first choice would be to stay on the A82, and travel down the west bank of Loch Lomond. Just after you come through Dumbarton, cross the Erskine Bridge (toll), and pick up the M8 on the south side of the River Clyde. Follow the M8 back to Edinburgh.
Option number two would be to turn left on the A85 at Crainlarich, and follow it to Lochearnhead. From there, go straight ahead on to the A84, which will take you through Callander, and on to Stirling. Stirling is well worth a visit, if you have the time. If not, you'll meet up with the M9, which will take you back to Edinburgh by way of Grangemouth.
Except for a small section of the A82, and the A87 on Skye, you won't be backtracking at all, and you'll see some of the finest scenery in Scotland outside of the Northwest Highlands. You'll also be on good roads all the way, with the exception of some single track roads on Skye, if you decide to take them. Believe me, you won't find anything like the I-5!
Have a safe and wonderful trip, and don't forget, you'll be driving on the left, and shifting gears with your left hand!