We don't have a full understanding of episodic recall, but let me try to answer from just a theoretical perspective (i.e., how could episodic memories be recalled without a start/end flag): an episodic memory is a sequence of brain states that correspond to previously experienced states.
The sequence occurs because connectivity promotes the network to progress from State A to State B to State C, etc. Additionally, all of the states consisting of that one "connected" memory are also associated, so that they prime one another.
To recall, you don't need to start at State A - you can start anywhere in the chain, and just recalling State C will prime you to also recall State A and B, so you can mentally "rewind" the memory a bit (we have no idea how exactly this type of conscious control works), even though it progresses most clearly in the A->B->C sequence.
The point of entry that allows you to initially get in to this chain might not be part of that episodic memory, either. For example, maybe you see a cat, and this leads to to an episodic memory of one event you shared with your own cat 5 years ago.
Brains are very dynamic, they do not behave like a feed-forward computation you might implement in a simple computer program.