A lot is said about the importance of beginnings and endings, but the middle is vital too. It's no good having an arresting opening and then it all sinks like an ill-mixed sponge cake in the middle. Then the reader won't reach the wonderful ending. And the brilliant beginning will be forgotten. It's awful to promise much, but deliver little. Far better to have an intriguing start from which a Pandora's box of delights is revealed as you read on, before coming to the great exciting final paragraphs.
When the middle is finished, you might chop some of the opening. It may seem far too banal once you have found out just how thrilling the rest of the story is. However, the writer who plans in advance might be more aware of the forthcoming action, so his beginnings will be more appropriate. But I write as I go, so I never know if the start will be any good until I've finished the middle.
The middle is sometimes frustrating, as you realise there are different ways in which the story can go. I started one that was destined for a women's magazine, but it is now unsuitable for that market because, once I'd reached the middle section, it was off and running in a totally different direction altogether. it will probably be sent to a competition instead.
See where the middle takes you, but never let the interest flag. Adjust the opening to suit the rest. Never forget your theme - that is absolutely essential and will keep you on the right path.