Vabsie,
Welcome to the wide world of aviation.
Most light airplanes have a glide ration which approximates about a 6:1 ratio or so, though some as little as 3:1...for every foot the aircraft descends, it glides forward three to six feet. Some better, some worse, but that's about the average. That said, the distance one can glide forward isn't particularly important. What's important is being able to land once you get done with the glide.
I've experienced a number of engine failures over the years in single and multi engine aircraft. Only one has resulted in any damage, and that occured at low level in mountainous terrain in the middle of a forest fire. The damage was minor, and I was flying the same airplane a month later after thorough inspections and some light repairs. That particular airplane had a relatively inefficient glide ratio, but the distance wasn't the issue; it was the landing, which was made on rough ground on a hillside. It ended well; I walked away.
Remember that the airplane flies with or without the engine; you don't fly the engine, you fly the wing. Excess thrust, or power, determines the ability of the airplane to climb...but not to land. Only you can do that.
Your safety depends largely on how you plan and conduct your flight. Remaining over and within distance of suitable forced landing sites, for example, is an important part of operating the airplane safely; always know where you can go and have a plan to get there, no matter what the altitude.
I've spent a lot of time in single engine airplanes in places that aren't conducive to a forced landing; inside the Grand Canyon, for example, where very few long, flat spaces are available to set down in the event of an engine failure. I did nearly all my flying there in single engine piston airplanes, often down inside the canyon, often landing on dirt runways and operating in box canyons in rocky, sheer terrain.
In such cases, glide ratio and the best glider was never an issue. The ability to put the airplane down was, and it did happen from time to time to individuals who experienced mechanical failues, fuel issues, weather, conditions that no longer permitted flight (extremely strong downdrafts, etc) and so forth. I experienced one engine failure in there during my time. What I did do was ensure that while a long flat smooth surface might not be available, I could always make a controlled forced landing if the need would arise; down river, on a sandbar, in the water, into trees, etc. I used local wind currents (such as strong updrafts on the downwind side of the canyon) to my benifit and planned my flight through terrain to take advantage of both the terrain and the conditions to give me the best opportunities in the event such a situation would arise.
Most importantly one must always recognize the concept that an engine failure is never a matter of if, but when. It's a mechanical object, just like the airplane, and mechanical objects break. I never make a takeoff without planning for an engine failure. In comparison to the number of takeoffs I've made, the number of engine failures is a very small percentage...very, very small. But it does happen, it has happened, and I always plan on it, however remote the possibility may be.
A fairly recent event in the news was a 747 which experienced multiple engine failures or power loss shortly after takeoff from a mountain field at night, and executed a very dramatic, though argueably successful, forced landing off field. The aircraft was completely destroyd, except for the flight deck, in which eight crewmembers survived. The point there is that while the airplane didn't have what one might call a favorable glide ratio, and certainly isn't made for off-field forced landings, one was made...and most importantly, no matter how many engines you may have, you should always plan on losing them. It does happen.
In a light airplane, power loss isn't the end of the world. The most common engine failure isn't a complete, catastrophic failure, but a partial-power failure. This may or may not enable you to return to an airport. A failed cylinder, for example, or even a fouled spark plug, slipped magneto, etc...might cause enough shaking and vibration to require shutting the engine down, or may simply enable you to switch the good mag and return home.
Whether the engine runs or doesn't, the airplane still flies, and your only concern is maintaining enough airspeed to maintain control while switching your plans to put the airplane on the ground.
North Atlantic flights in single engine airplanes are gambles, no matter how you slide it.