How far is Cuba?

Depends on where you are, yuk yuk!

Great Fishing in
Golfo de Guahanacabibes

Ok, seriously - from Key West, it's 95 nm - nautical miles, or about 110 statute miles. From Islamorada Mile Five channel bridge, about 115 nm. From Ragged Island, in the Bahamas, it's 65 nm and from Isla Mujeres, it's around 80nm. So it's close, even in sailing terms. What's more important though, is how you sail from where you are. You have to figure out the currents, especially the Gulf Stream if you're crossing from the Keys, the winds and weather...get it right and it's an easy trip. Get it wrong and you'll be wishing you'd bought an RV and were headed for New Mexico. The chartlet on the first page of this blog shows several routes to and from Cuba. Each has advantages, and disadvantages.

Club Nautico, Marina Hemingway, Havana

When it's possible, you want to travel with the currents and the wind behind you. Worst case scenario is traveling with the wind against the current. That heaps up big, short waves that can be nasty. Unfortunately, when crossing from the Keys, you're often faced with big beam seas as the wind, usually out of the east, is up against the Gulf Stream, running eastward. One trick I and others have successfully used is to come across on the tail end of a norther, when the winds are out of the northwest and dying. If you're sailing into Varadero or Havana though, this wind direction can bring big seas into the entrance channels. You may have to stand off until the wind is into the northeast to enter these locations.
Best is, and call me a coward, to catch a nice high pressure area and cross under power, with minimal wind and waves. Sure, it's not 'sailing', but it gets you there safely and without drama. Of course, then you don't get to see sights such as dozens of dolphins, the smaller black variety, leaping from one wave into the next as they pace your boat through the waves. Unforgettable. I'd have taken photos, but I was way too busy sailing the boat.