You can drive from Edinburgh to Fort William, via Glencoe in about 3½ hours - so there and back in a day is easy. The only real downside is that, if you don't leave until, say, 8:00am and want to get back before midnight, then you are going to miss the best light at dawn and dusk.
Alternatively, you can get to one of the "gateways to the Highlands" at Aberfoyle or Pitlochry in 2 hours from Edinburgh.
Don't worry about seeing it all in one trip. I have lived in Scotland for 68 years and doubt if I have seen 50% of the country.