I'm always surprised at the problems folks have with this but regardless of that I can say with some assurance that a patched tube is not particularly prone to any leakage. I have tubes that are in use for years with patches on them. Sometimes multiple patches. A good scuffing with the sand paper, a light application of the glue that is allowed to dry for 2 or 3 min (no more is needed) and then a firm application of the patch and you're done. I never carry a spare tube unless there is some big group or I'm doing a century. If you fail to get the edge glued you can lift it a bit, rub a bit of glue in there...try to give it a minute or so to dry out and press down on it. If the hole is already covered by the patch it won't make much difference. If you don't let the glue dry (it will not be entirely dry....but you can see the reflective shine become dull after a minute or so) the patch won't stick properly. A properly patched tire will not leak from the patch and the patch will last as long as the tube. Once a tube has three or four patches I'm apt to toss it for no particular reason but once you're comfortable patching a tube you'll have complete confidence in patches. One good reason to bring spare tubes is rain. If it's pouring out it can be a challenge to get the tube dry and properly patched. When mosquitos are bad here in Alaska I also may be more apt to bring a spare tube as I've patched tires while being eaten alive by mosquitos and that is no fun. Short of that....just patch and pedal.