As the other members have pointed out, kendo or other martial arts are a great source of experience. Even better if there's a HEMA-group, as they specifically deal in european swordfighting. "Modern" sport-fencing usually isn't a very good substitute, since it's a much faster and points-only oriented type of battle. The blades are very light, and if it where actual combat, it would kill BOTH participants 9 times out of 10. It only comes down to who hit that 1/100th of a second earlier. Good for reflex-training though.
If you don't have any clubs conveniently nearby, make some mock-up swords out of bound wood and go bashing your friends! It's great fun, but also very educational if you have a well-made replica and try to actually form strategies to learn from it. A good cross-guard is central to many longsword techniques, so definitely put in the effort of making that. Otherwise, just keep the weight and balance reasonably close and you'll have a great training-tool.