The one downside of folding bikes is they are usually smaller tires which means not as efficient over bumps and such so you need to practice and train accordingly.

If your used to riding and in shape 50 miles is doable, if your out of shape its not going to be easy. I've been busy with other things for the last few years and finally got on my bike last summer and rode 15 miles in about two hours and was tired and sore for a week.

Also mountain vs road vs hybrid bike, the mountain tires have a lot more rolling resistance so its a lot more work for the miles. That being said for training and getting back into shape I'm using my mountain bike and those tires work as more resistance to help build up strength and endurance.